SDG #031 GPSDO & Clock Update - BST Ends

The end of British Summer Time is a genuinely useful annual test for any GPS-referenced clock project, and that is exactly what this short update covers. A GPSDO (GPS disciplined oscillator) recovers time-of-day information from the satellite constellation alongside its frequency reference, but GPS itself broadcasts UTC, not local time. Any daylight saving behaviour has to be implemented in firmware, which means the interesting question is not whether the timekeeping is accurate but whether the timezone and DST logic was written correctly and actually fires on the right Sunday at the right hour.

This is worth understanding before buying or building a GPS clock. Many cheap modules and displays either ignore DST entirely, expect you to set an offset manually twice a year, or apply a hardcoded US-style rule that shifts on the wrong dates for UK and European users. A unit that rolls back automatically at 02:00 local on the last Sunday of October is doing real work; one that needs manual intervention is really just a very accurate UTC clock with a knob on it.

The broader takeaway is that GPSDO accuracy claims and clock usability are separate things. The oscillator side gives you a traceable frequency reference for test gear calibration, while the displayed time depends entirely on the software layer sitting on top of it. If you are evaluating a GPSDO or a homebrew clock build, check the DST handling and the timezone configuration before assuming the display will look after itself.