Someone else will likely chime in but I recall seeing some problems mentioned in the forums with the clock on iOS (newer versions of the app). I'm not finding anything with a quick search but will link it if I find something.
Clock Problems
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Played two 5-minute games this morning and lost on time. I'm using the iPad iOS app.
I was killing on time the first game and had plenty of time left when opponent was down to less than 30 seconds. The next thing I know I'm out of time.
The 2nd game I played, both me and my opponent were moving very quickly through the opening 3 moves. I look down and I'm down to 4:38. I know I took less than 10 seconds for my moves.
I watched as my clock clicked down at a fast rate throughtout the game. It looked like it was going down 2 seconds for every 1 second of actual time.
About 2 weeks ago I notices the same behaviour. Makes playing a 5-minute game next to impossible as it is being timed at almost a 2-minute rate.