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<title>Case Study: ISO 8601 Date + Time — Fuzzing with Fandango</title>
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<div id="jb-print-docs-body" class="onlyprint">
<h1>Case Study: ISO 8601 Date + Time</h1>
<!-- Table of contents -->
<div id="print-main-content">
<div id="jb-print-toc">
<div>
<h2> Contents </h2>
</div>
<nav aria-label="Page">
<ul class="visible nav section-nav flex-column">
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#creating-grammars-programmatically">Creating Grammars Programmatically</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#spec-header">Spec Header</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#date">Date</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#calendar-dates">Calendar Dates</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#months">Months</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#days">Days</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#week-dates">Week Dates</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#ordinal-dates">Ordinal Dates</a></li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#time">Time</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#hours">Hours</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#minutes">Minutes</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#seconds">Seconds</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#time-zones">Time Zones</a></li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#ensuring-validity">Ensuring Validity</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#even-better-validity">Even Better Validity</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#fuzzing-dates-and-times">Fuzzing Dates and Times</a></li>
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<article class="bd-article">
<section class="tex2jax_ignore mathjax_ignore" id="case-study-iso-8601-date-time">
<span id="sec-iso8601"></span><h1>Case Study: ISO 8601 Date + Time<a class="headerlink" href="#case-study-iso-8601-date-time" title="Link to this heading">#</a></h1>
<p>Let us make use of what we have explored so far in a more elaborate case study.
<a class="reference external" href="https://en.wikipedia.org/wiki/ISO_8601">ISO 8601</a> is an international standard for exchange and communication of <em>date and time-related data</em>, providing an unambiguous method of representing calendar dates and times.</p>
<p>In this chapter, we will define a full Fandango spec for ISO 8601 date and time formats, ensuring both syntactic and semantic validity.
To make matters more interesting, we will create the spec <em>programmatically</em> - that is, by having a number of Python functions that generate the spec for us.</p>
<section id="creating-grammars-programmatically">
<h2>Creating Grammars Programmatically<a class="headerlink" href="#creating-grammars-programmatically" title="Link to this heading">#</a></h2>
<p>We start with a number of functions that help us create fragments of the grammar.</p>
<p><code class="docutils literal notranslate"><span class="pre">make_rule()</span></code> creates a grammar rule from <code class="docutils literal notranslate"><span class="pre">symbol</span></code> and its possible <code class="docutils literal notranslate"><span class="pre">expansions</span></code>:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">sys</span>
<span class="k">def</span><span class="w"> </span><span class="nf">print_s</span><span class="p">(</span><span class="n">s</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">""</span><span class="p">)</span>
<span class="k">return</span> <span class="n">s</span>
<span class="k">def</span><span class="w"> </span><span class="nf">make_rule</span><span class="p">(</span><span class="n">symbol</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">expansions</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="nb">str</span><span class="p">],</span> <span class="n">sep</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">''</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="k">return</span> <span class="n">print_s</span><span class="p">(</span><span class="sa">f</span><span class="s2">"</span><span class="si">{</span><span class="n">sep</span><span class="si">}</span><span class="s2"><</span><span class="si">{</span><span class="n">symbol</span><span class="si">}</span><span class="s2">> ::= "</span> <span class="o">+</span> <span class="s2">" | "</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">expansions</span><span class="p">)</span> <span class="o">+</span> <span class="s2">"</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
<span class="n">make_rule</span><span class="p">(</span><span class="s2">"start"</span><span class="p">,</span> <span class="p">[</span><span class="s2">"<iso8601datetime>"</span><span class="p">]);</span> <span class="c1"># a final ";" suppresses result</span>
</pre></div>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <iso8601datetime>
</pre></div>
</div>
</div>
</div>
<p>We can also use <code class="docutils literal notranslate"><span class="pre">make_rule()</span></code> to quickly create a list of expansions:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">make_rule</span><span class="p">(</span><span class="s2">"digit"</span><span class="p">,</span> <span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">digit</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">digit</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">)]);</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><digit> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
</pre></div>
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</div>
</div>
<p>These additional functions help in adding non-grammar elements such as headers, code, and constraints:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">make_header</span><span class="p">(</span><span class="n">title</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="k">return</span> <span class="n">print_s</span><span class="p">(</span><span class="sa">f</span><span class="s2">"</span><span class="se">\n</span><span class="s2"># </span><span class="si">{</span><span class="n">title</span><span class="si">}</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
<span class="k">def</span><span class="w"> </span><span class="nf">make_comment</span><span class="p">(</span><span class="n">comment</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">sep</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">''</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="k">return</span> <span class="n">print_s</span><span class="p">(</span><span class="sa">f</span><span class="s2">"</span><span class="si">{</span><span class="n">sep</span><span class="si">}</span><span class="s2"># </span><span class="si">{</span><span class="n">comment</span><span class="si">}</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
<span class="k">def</span><span class="w"> </span><span class="nf">make_constraint</span><span class="p">(</span><span class="n">constraint</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="k">return</span> <span class="n">print_s</span><span class="p">(</span><span class="sa">f</span><span class="s2">"where </span><span class="si">{</span><span class="n">constraint</span><span class="si">}</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
<span class="k">def</span><span class="w"> </span><span class="nf">make_code</span><span class="p">(</span><span class="n">code</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="k">return</span> <span class="n">print_s</span><span class="p">(</span><span class="sa">f</span><span class="s2">"</span><span class="si">{</span><span class="n">code</span><span class="si">}</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
<span class="n">make_header</span><span class="p">(</span><span class="s2">"ISO 8601 grammar"</span><span class="p">);</span>
</pre></div>
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</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span># ISO 8601 grammar
</pre></div>
</div>
</div>
</div>
</section>
<section id="spec-header">
<h2>Spec Header<a class="headerlink" href="#spec-header" title="Link to this heading">#</a></h2>
<p>Let us create a Fandango spec.
For now, we store the final spec in the <code class="docutils literal notranslate"><span class="pre">iso8601lib</span></code> string variable, to be saved in a file at the end.
Every time we add a new fragment, the new fragment will be output here, so we can see the actual content of <code class="docutils literal notranslate"><span class="pre">iso8601lib</span></code> incrementally.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">=</span> <span class="n">make_header</span><span class="p">(</span><span class="s2">"ISO 8601 date and time"</span><span class="p">)</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_comment</span><span class="p">(</span><span class="s2">"Generated by docs/ISO8601.md. Do not edit."</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span># ISO 8601 date and time
# Generated by docs/ISO8601.md. Do not edit.
</pre></div>
</div>
</div>
</div>
<p>We will need the <code class="docutils literal notranslate"><span class="pre">datetime</span></code> library below, so we import it.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_code</span><span class="p">(</span><span class="s2">"</span><span class="se">\n</span><span class="s2">import datetime</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
</pre></div>
</div>
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<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>import datetime
</pre></div>
</div>
</div>
</div>
</section>
<section id="date">
<h2>Date<a class="headerlink" href="#date" title="Link to this heading">#</a></h2>
<p>We start with a <code class="docutils literal notranslate"><span class="pre"><start></span></code> symbol.
An ISO 8601 date/time spec starts with a date, and an optional time, separated by <code class="docutils literal notranslate"><span class="pre">T</span></code>:</p>
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<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"start"</span><span class="p">,</span> <span class="p">[</span><span class="s2">"<iso8601datetime>"</span><span class="p">])</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601datetime"</span><span class="p">,</span>
<span class="p">[</span><span class="s2">"<iso8601date> ('T' <iso8601time>)?"</span><span class="p">])</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <iso8601datetime>
<iso8601datetime> ::= <iso8601date> ('T' <iso8601time>)?
</pre></div>
</div>
</div>
</div>
<p>A date can either be a <em>calendar date</em>, but also a <em>week date</em> or an <em>ordinal date</em>.</p>
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<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601date"</span><span class="p">,</span>
<span class="p">[</span><span class="s2">"<iso8601calendardate>"</span><span class="p">,</span>
<span class="s2">"<iso8601weekdate>"</span><span class="p">,</span>
<span class="s2">"<iso8601ordinaldate>"</span><span class="p">],</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601date> ::= <iso8601calendardate> | <iso8601weekdate> | <iso8601ordinaldate>
</pre></div>
</div>
</div>
</div>
<section id="calendar-dates">
<h3>Calendar Dates<a class="headerlink" href="#calendar-dates" title="Link to this heading">#</a></h3>
<p>An ISO 8601 calendar date has the format YYYY-MM-DD, but can also be YYYY-MM or YYYYMMDD.
The year can be prefixed with <code class="docutils literal notranslate"><span class="pre">+</span></code> or <code class="docutils literal notranslate"><span class="pre">-</span></code> to indicate <a class="reference external" href="https://en.wikipedia.org/wiki/Anno_Domini">AC/BC</a> (or <a class="reference external" href="https://en.wikipedia.org/wiki/Common_Era">CE/BCE</a>) designators.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601calendardate"</span><span class="p">,</span>
<span class="p">[</span><span class="s2">"<iso8601year> '-' <iso8601month> ('-' <iso8601day>)?"</span><span class="p">,</span>
<span class="s2">"<iso8601year> <iso8601month> <iso8601day>"</span><span class="p">],</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601year"</span><span class="p">,</span> <span class="p">[</span><span class="s2">"('+'|'-')? <digit></span><span class="si">{4}</span><span class="s2">"</span><span class="p">])</span>
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</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601calendardate> ::= <iso8601year> '-' <iso8601month> ('-' <iso8601day>)? | <iso8601year> <iso8601month> <iso8601day>
<iso8601year> ::= ('+'|'-')? <digit>{4}
</pre></div>
</div>
</div>
</div>
</section>
<section id="months">
<h3>Months<a class="headerlink" href="#months" title="Link to this heading">#</a></h3>
<p>Months are easy:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601month"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">month</span><span class="si">:</span><span class="s2">02d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">month</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">13</span><span class="p">)])</span>
</pre></div>
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<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601month> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12'
</pre></div>
</div>
</div>
</div>
</section>
<section id="days">
<h3>Days<a class="headerlink" href="#days" title="Link to this heading">#</a></h3>
<p>As with months, we define each day individually to avoid biasing the grammar.
The fact that some months have only 28, 29, or 30 days will be handled later on in a constraint.</p>
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<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601day"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">day</span><span class="si">:</span><span class="s2">02d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">day</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">32</span><span class="p">)])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601day> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31'
</pre></div>
</div>
</div>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>For testing purposes, it makes sense to test with <em>extreme</em> values - in our case, January 1, February 28 or 29, and December 31.</p>
</div>
</section>
<section id="week-dates">
<h3>Week Dates<a class="headerlink" href="#week-dates" title="Link to this heading">#</a></h3>
<p>ISO 8601 also allows specifying dates by week numbers (1 - 53) and optional days of the week (1 - 7).
<code class="docutils literal notranslate"><span class="pre">2025-W12-1</span></code> is the Monday of the 12th week in 2025.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601weekdate"</span><span class="p">,</span>
<span class="p">[</span><span class="s2">"<iso8601year> '-'? 'W' <iso8601week> ('-' <iso8601weekday>)?"</span><span class="p">],</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601week"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">week</span><span class="si">:</span><span class="s2">02d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">week</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">54</span><span class="p">)])</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601weekday"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">weekday</span><span class="si">:</span><span class="s2">1d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">weekday</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">8</span><span class="p">)])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601weekdate> ::= <iso8601year> '-'? 'W' <iso8601week> ('-' <iso8601weekday>)?
<iso8601week> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53'
<iso8601weekday> ::= '1' | '2' | '3' | '4' | '5' | '6' | '7'
</pre></div>
</div>
</div>
</div>
</section>
<section id="ordinal-dates">
<h3>Ordinal Dates<a class="headerlink" href="#ordinal-dates" title="Link to this heading">#</a></h3>
<p>ISO 8601 also allows specifying dates by day numbers (1 - 366).
<code class="docutils literal notranslate"><span class="pre">2000-366</span></code>, for instance, specifies the last day in 2000 (which was a leap year).
Again, we specify each day programmatically (and individually).</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601ordinaldate"</span><span class="p">,</span>
<span class="p">[</span><span class="s2">"<iso8601year> ('-'? <iso8601ordinalday>)?"</span><span class="p">],</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601ordinalday"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">day</span><span class="si">:</span><span class="s2">03d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">day</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">367</span><span class="p">)])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601ordinaldate> ::= <iso8601year> ('-'? <iso8601ordinalday>)?
<iso8601ordinalday> ::= '001' | '002' | '003' | '004' | '005' | '006' | '007' | '008' | '009' | '010' | '011' | '012' | '013' | '014' | '015' | '016' | '017' | '018' | '019' | '020' | '021' | '022' | '023' | '024' | '025' | '026' | '027' | '028' | '029' | '030' | '031' | '032' | '033' | '034' | '035' | '036' | '037' | '038' | '039' | '040' | '041' | '042' | '043' | '044' | '045' | '046' | '047' | '048' | '049' | '050' | '051' | '052' | '053' | '054' | '055' | '056' | '057' | '058' | '059' | '060' | '061' | '062' | '063' | '064' | '065' | '066' | '067' | '068' | '069' | '070' | '071' | '072' | '073' | '074' | '075' | '076' | '077' | '078' | '079' | '080' | '081' | '082' | '083' | '084' | '085' | '086' | '087' | '088' | '089' | '090' | '091' | '092' | '093' | '094' | '095' | '096' | '097' | '098' | '099' | '100' | '101' | '102' | '103' | '104' | '105' | '106' | '107' | '108' | '109' | '110' | '111' | '112' | '113' | '114' | '115' | '116' | '117' | '118' | '119' | '120' | '121' | '122' | '123' | '124' | '125' | '126' | '127' | '128' | '129' | '130' | '131' | '132' | '133' | '134' | '135' | '136' | '137' | '138' | '139' | '140' | '141' | '142' | '143' | '144' | '145' | '146' | '147' | '148' | '149' | '150' | '151' | '152' | '153' | '154' | '155' | '156' | '157' | '158' | '159' | '160' | '161' | '162' | '163' | '164' | '165' | '166' | '167' | '168' | '169' | '170' | '171' | '172' | '173' | '174' | '175' | '176' | '177' | '178' | '179' | '180' | '181' | '182' | '183' | '184' | '185' | '186' | '187' | '188' | '189' | '190' | '191' | '192' | '193' | '194' | '195' | '196' | '197' | '198' | '199' | '200' | '201' | '202' | '203' | '204' | '205' | '206' | '207' | '208' | '209' | '210' | '211' | '212' | '213' | '214' | '215' | '216' | '217' | '218' | '219' | '220' | '221' | '222' | '223' | '224' | '225' | '226' | '227' | '228' | '229' | '230' | '231' | '232' | '233' | '234' | '235' | '236' | '237' | '238' | '239' | '240' | '241' | '242' | '243' | '244' | '245' | '246' | '247' | '248' | '249' | '250' | '251' | '252' | '253' | '254' | '255' | '256' | '257' | '258' | '259' | '260' | '261' | '262' | '263' | '264' | '265' | '266' | '267' | '268' | '269' | '270' | '271' | '272' | '273' | '274' | '275' | '276' | '277' | '278' | '279' | '280' | '281' | '282' | '283' | '284' | '285' | '286' | '287' | '288' | '289' | '290' | '291' | '292' | '293' | '294' | '295' | '296' | '297' | '298' | '299' | '300' | '301' | '302' | '303' | '304' | '305' | '306' | '307' | '308' | '309' | '310' | '311' | '312' | '313' | '314' | '315' | '316' | '317' | '318' | '319' | '320' | '321' | '322' | '323' | '324' | '325' | '326' | '327' | '328' | '329' | '330' | '331' | '332' | '333' | '334' | '335' | '336' | '337' | '338' | '339' | '340' | '341' | '342' | '343' | '344' | '345' | '346' | '347' | '348' | '349' | '350' | '351' | '352' | '353' | '354' | '355' | '356' | '357' | '358' | '359' | '360' | '361' | '362' | '363' | '364' | '365' | '366'
</pre></div>
</div>
</div>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>For testing purposes, it makes sense to test with <em>extreme</em> values - in our case, <code class="docutils literal notranslate"><span class="pre">1</span></code>, <code class="docutils literal notranslate"><span class="pre">365</span></code> and <code class="docutils literal notranslate"><span class="pre">366</span></code>.</p>
</div>
</section>
</section>
<section id="time">
<h2>Time<a class="headerlink" href="#time" title="Link to this heading">#</a></h2>
<p>Now for <em>time</em> specifications.
In ISO 8601, time is specified as <code class="docutils literal notranslate"><span class="pre">HH:MM:SS</span></code>, where <code class="docutils literal notranslate"><span class="pre">HH</span></code> comes in 24-hour format.
<code class="docutils literal notranslate"><span class="pre">MM</span></code> and <code class="docutils literal notranslate"><span class="pre">SS</span></code> are optional, as is the colon separator in an “unambiguous” context.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601time"</span><span class="p">,</span>
<span class="p">[</span><span class="s2">"'T'? <iso8601hour> (':'? <iso8601minute> (':'? <iso8601second> (('.' | ',') <iso8601fraction>)? )? )? <iso8601timezone>?"</span><span class="p">],</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601time> ::= 'T'? <iso8601hour> (':'? <iso8601minute> (':'? <iso8601second> (('.' | ',') <iso8601fraction>)? )? )? <iso8601timezone>?
</pre></div>
</div>
</div>
</div>
<section id="hours">
<h3>Hours<a class="headerlink" href="#hours" title="Link to this heading">#</a></h3>
<p>Hours normally go from <code class="docutils literal notranslate"><span class="pre">00</span></code> to <code class="docutils literal notranslate"><span class="pre">23</span></code>, but <code class="docutils literal notranslate"><span class="pre">24:00:00</span></code> is allowed to represent midnight at the end of a day.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_comment</span><span class="p">(</span><span class="s2">"24:00:00 is allowed to represent midnight at the end of a day"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601hour"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">hour</span><span class="si">:</span><span class="s2">02d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">hour</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">25</span><span class="p">)])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span># 24:00:00 is allowed to represent midnight at the end of a day
<iso8601hour> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24'
</pre></div>
</div>
</div>
</div>
</section>
<section id="minutes">
<h3>Minutes<a class="headerlink" href="#minutes" title="Link to this heading">#</a></h3>
<p>Minutes go from <code class="docutils literal notranslate"><span class="pre">00</span></code> to <code class="docutils literal notranslate"><span class="pre">59</span></code>.
Nothing special here.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601minute"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">minute</span><span class="si">:</span><span class="s2">02d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">minute</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">60</span><span class="p">)])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601minute> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53' | '54' | '55' | '56' | '57' | '58' | '59'
</pre></div>
</div>
</div>
</div>
</section>
<section id="seconds">
<h3>Seconds<a class="headerlink" href="#seconds" title="Link to this heading">#</a></h3>
<p>Seconds normally go from <code class="docutils literal notranslate"><span class="pre">00</span></code> to <code class="docutils literal notranslate"><span class="pre">59</span></code>, but <code class="docutils literal notranslate"><span class="pre">60</span></code> can be used to represent leap seconds.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_comment</span><span class="p">(</span><span class="s2">"xx:yy:60 is allowed to represent leap seconds"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601second"</span><span class="p">,</span>
<span class="p">[</span><span class="sa">f</span><span class="s2">"'</span><span class="si">{</span><span class="n">second</span><span class="si">:</span><span class="s2">02d</span><span class="si">}</span><span class="s2">'"</span> <span class="k">for</span> <span class="n">second</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">61</span><span class="p">)])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span># xx:yy:60 is allowed to represent leap seconds
<iso8601second> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53' | '54' | '55' | '56' | '57' | '58' | '59' | '60'
</pre></div>
</div>
</div>
</div>
<p>Seconds can be followed by <code class="docutils literal notranslate"><span class="pre">,</span></code> or a <code class="docutils literal notranslate"><span class="pre">.</span></code> (see <code class="docutils literal notranslate"><span class="pre"><iso8601time></span></code>, above) and a fraction - an arbitrary number of digits.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601fraction"</span><span class="p">,</span> <span class="p">[</span><span class="s2">"<digit>+"</span><span class="p">])</span>
</pre></div>
</div>
</div>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601fraction> ::= <digit>+
</pre></div>
</div>
</div>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Testing with <em>extreme</em> values would mandate times such as <code class="docutils literal notranslate"><span class="pre">20:15:00,99999999999999999999</span></code> to test for possible buffer overflows.</p>
</div>
</section>
<section id="time-zones">
<h3>Time Zones<a class="headerlink" href="#time-zones" title="Link to this heading">#</a></h3>
<p>Any time can be followed by a time zone designator.
This is either <code class="docutils literal notranslate"><span class="pre">Z</span></code>, indicating Coordinated Universal Time (UTC), or an offset designating the time zone.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_rule</span><span class="p">(</span><span class="s2">"iso8601timezone"</span><span class="p">,</span> <span class="p">[</span><span class="s2">"'Z'"</span><span class="p">,</span>
<span class="s2">"'+' <iso8601hour> (':'? <iso8601minute>)?"</span><span class="p">,</span>
<span class="s2">"'-' <iso8601hour> (':'? <iso8601minute>)?"</span><span class="p">],</span> <span class="n">sep</span><span class="o">=</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><iso8601timezone> ::= 'Z' | '+' <iso8601hour> (':'? <iso8601minute>)? | '-' <iso8601hour> (':'? <iso8601minute>)?
</pre></div>
</div>
</div>
</div>
</section>
</section>
<section id="ensuring-validity">
<h2>Ensuring Validity<a class="headerlink" href="#ensuring-validity" title="Link to this heading">#</a></h2>
<p>So far, our dates and times are <em>syntactically</em> valid, but not necessarily <em>semantically</em>.
We can easily create invalid dates such as <code class="docutils literal notranslate"><span class="pre">2025-02-31</span></code> (February 31) or invalid times such as <code class="docutils literal notranslate"><span class="pre">24:10:00</span></code> (10 minutes past midnight - only <code class="docutils literal notranslate"><span class="pre">24:00:00</span></code> is allowed).
We could now add a number of additional <em>rules</em> to check for all these properties, and/or extend the grammar accordingly.
However, we can also simply be lazy and have the existing Python <code class="docutils literal notranslate"><span class="pre">datetime</span></code> module check all this for us:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">datetime</span>
</pre></div>
</div>
</div>
</div>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">is_valid_iso8601datetime</span><span class="p">(</span><span class="n">iso8601datetime</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">bool</span><span class="p">:</span>
<span class="w"> </span><span class="sd">"""Return True iff `iso8601datetime` is valid."""</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">fromisoformat</span><span class="p">(</span><span class="n">iso8601datetime</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">False</span>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">is_valid_iso8601datetime</span><span class="p">(</span><span class="s2">"2025-01-01T14:00"</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>True
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">is_valid_iso8601datetime</span><span class="p">(</span><span class="s2">"2025-02-31"</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>False
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">is_valid_iso8601datetime</span><span class="p">(</span><span class="s2">"2000-02-29T00:00:00"</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>True
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<p>With this, we can now add a <em>constraint</em> that limits our generator to only valid dates and times:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="n">make_constraint</span><span class="p">(</span><span class="s2">"is_valid_iso8601datetime(str(<iso8601datetime>))"</span><span class="p">)</span>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>where is_valid_iso8601datetime(str(<iso8601datetime>))
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">iso8601lib</span> <span class="o">+=</span> <span class="s1">'''</span>
<span class="s1">def is_valid_iso8601datetime(iso8601datetime: str) -> bool:</span>
<span class="s1"> """Return True iff `iso8601datetime` is valid."""</span>
<span class="s1"> try:</span>
<span class="s1"> datetime.datetime.fromisoformat(iso8601datetime)</span>
<span class="s1"> return True</span>
<span class="s1"> except ValueError:</span>
<span class="s1"> return False</span>
<span class="s1">'''</span>
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</section>
<section id="even-better-validity">
<h2>Even Better Validity<a class="headerlink" href="#even-better-validity" title="Link to this heading">#</a></h2>
<p>The Python <code class="docutils literal notranslate"><span class="pre">datetime</span></code> module has a some limitations, which extend to our spec.
For instance, <code class="docutils literal notranslate"><span class="pre">datetime</span></code> does not support <code class="docutils literal notranslate"><span class="pre">24:00:00</span></code> as a valid time:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">is_valid_iso8601datetime</span><span class="p">(</span><span class="s2">"2024-12-31T24:00:00"</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>False
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<p>Hence, our <code class="docutils literal notranslate"><span class="pre">iso8601.fan</span></code> spec may miss out a number of ISO 8601 features.</p>
<p>The <a class="reference external" href="https://dateutil.readthedocs.io/en/stable/index.html"><code class="docutils literal notranslate"><span class="pre">dateutil</span></code> alternative</a> provides an ISO 8601 parser without these deficiencies.</p>
<p>Let us redefine <code class="docutils literal notranslate"><span class="pre">is_valid_iso8601datetime()</span></code> to make use of the <code class="docutils literal notranslate"><span class="pre">dateutil</span></code> parser:</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">is_valid_iso8601datetime</span><span class="p">(</span><span class="n">iso8601datetime</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">bool</span><span class="p">:</span>
<span class="w"> </span><span class="sd">"""Return True iff `iso8601datetime` is valid."""</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">dateutil</span><span class="o">.</span><span class="n">parser</span><span class="o">.</span><span class="n">isoparse</span><span class="p">(</span><span class="n">iso8601datetime</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">False</span>
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<p>Let us see if the above example now works:</p>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>True
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<p>This now works! How about the earlier examples?</p>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>True
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">is_valid_iso8601datetime</span><span class="p">(</span><span class="s2">"2025-02-31"</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>False
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">is_valid_iso8601datetime</span><span class="p">(</span><span class="s2">"2000-02-29T00:00:00"</span><span class="p">)</span>
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<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>True