You can archive a timer on the app by swiping it from right to left, then tapping the "Archive" button that is revealed. When you archive a timer, it does not affect any time entries that you already created with that timer.

To restore a timer, tap in the white are where the name of your timer is displayed, then tap “Restore archived timers” at the bottom of the screen. Find the timer you want and tap the Restore button.