I have had the same issue with my Forerunner 35. I have tried changing the settings in the Garmin Connect app - set to 24 hour time format and move alert bar to off - and that seems to have settled things down - so far anyway!
Joe, thank you so much for your reply. Since posting my problem I have realised that the phone overrides the watch. It was frustrating at the time, but obvious in retrospect. Thanks again Joe for taking the time to reply.
Jim
My phone time is also 25 hour format and correct time, but my watch continues to revert back to 12 hour
My phone time is also 25 hour format and correct time
It's not the phone time format that matters. It's the setting in Garmin Connect. Go to your Garmin Connect mobile app, and check the time format in the device settings.