Why Garmin 945 lose time when set to manual time? And how to prevent?

I’ve got a new Garmin 945. Sometimes where I live, my cellphone switches timezones because we are close to the border, so around here, we always keep them on manual. I was thinking it would be good to do the same thing on my Garmin watch because after the first day, I caught it in the wrong timezone. I set the watch to manual and then one morning was mystified that it was about 20 minutes off the real time. Reset it and a couple days later it has happened again. Any other experience with this one?