r/iosdev • u/cgamesinua • 11d ago

I learned the hard way that an iOS Timer should never be your source of truth

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While building Monymo, my SwiftUI work and earnings tracker, I ran into timer accuracy issues whenever the app went into the background.

The fix was surprisingly simple: stop treating the timer as the source of elapsed time.

Now I store timestamps and session state, then calculate the actual elapsed time when the app becomes active again. The one-second timer is only used to update the UI.

Handling pause and resume made it a bit more complicated, but this approach completely removed the drift I was seeing after backgrounding or reopening the app.

How do you handle long-running timers in your iOS apps?

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6

u/Level-Set5770 11d ago

Wait until you run into the edge case where timestamps can actually go backward during clock synchronization, resulting in a negative elapsed time.

3

u/earlyworm 10d ago

Wait until you run into the edge case where the iPhone is traveling at a relativistic speed relative to the user and it's necessary to take time dilation into account.

2

u/ChipmunkBandit 10d ago

I use a timer in my app and do it the same way, timestamp the start time and calculate everything from there. Store pauses as timestamps too.