You can archive a timer on the app by clicking it, then clicking the "Archive" button that is revealed next to the "Done" button. When you archive a timer, it does not affect any time entries that you already created with that timer.

 

To restore a timer, tap “Add new timer” on the main screen (you may have to scroll to see it), then click “View archived timers” find the timer you want and tap the Restore Timer button.