New Sub C - Keeping Time

[Capeboss]

New Sub C owner here and have to say I'm a bit overwhelmed by all of the info on this forum. So my question.... My watch has been a bit all over the place with keeping time. I have used time.gov as my reference point. Some days, first few, it was running 20-30 seconds fast. In the last few days it has been running about 4-6 seconds fast. What's strange is that it will keep perfect time overnight, but seems to gain time during the day when I wear it. I do a lot of walking 4-5 miles per day? Could this be impacting the ability to keep time. My buddy has a Sub that is 18 years old and his was 3 seconds fast over a week. Should I call the AD where I got it? Do watches have a "break in" period.

Any thoughts would be appreciated!