Update pisetup.md

Add info box with instruction how to manually set the system time and synchronize the Witty Pi RTC time with the system time if no internet connection is available.

Author: maxsitt

docs/software/pisetup.md

@@ -248,6 +248,19 @@ the Witty Pi is used by default but can be easily exchanged with the PiJuice.
 
     ![Witty Pi Config Sync Time](assets/images/wittypi_config_time.png){ width="700" }
 
+    ??? question "No internet connection"
+
+        If you are connected to the RPi hotspot and therefore are not able to
+        get the network time due to the missing internet connection, first set
+        the RPi system time to the current date + time manually by running:
+
+        ``` bash
+        sudo date -s "2025-09-08 14:00:00"
+        ```
+
+        Then use the first option `1. Write system time to RTC` in the Witty Pi
+        configuration tool to synchronize the RTC time with the system time.
+
     ??? warning "Batteries Unplugged"
 
         If you unplug both the 3.7V battery and USB-C input (5V battery) from the