GPS delivers the time for automatic time and date, it needs to be in sync by the millisecond for GPS to work precisely, so it makes sense for the phone to use that data.
So i guess your phone has been accumulating more and more seconds in time delay as it relied on the far less precise internal quartz clock.
That being said, often you get up to 5 seconds less due to server lag, and you cannot really trust the last 10 seconds you have due to server lag, so you should aim to defeat the boss before the last 10 seconds start.
Keep in mind that if you used a raid pass for a raid, you can try as often as you like, you cannot try again if the time for the raid runs out or if you defeat the boss. If you fail and there is still time, heal up and try again, no further raid pass required.