The problem is that there are actually 3 different clocks: yours, your opponents, and the only one that counts, the server. Let me get a few articles for you...
https://support.chess.com/article/357-what-is-lag-in-live-chess
https://support.chess.com/article/423-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken
https://support.chess.com/article/213-how-do-i-fix-my-disconnect-lag-issues
There are more that you can access with Help (lower left corner) and then Ask a Question.
All previous threads in the forums are archived so you can also bring up previous discussions of this. Hope that was helpful.
I've been playing 5/5 bullet chess and I keep noticing toward the end of the game when the opponent has around .30 to 50 seconds left, all the sudden they gain a significant amount of time. Does 5 minutes not mean you have 5 minutes to use?