swift – iOS Background Mode for Timer Apps

I’m growing a timer app for iOS, much like the next apps:

These apps appear to maintain their timers operating even when the app is within the background.

My Questions:

1. Which Background Modes ought to be enabled to assist background timer performance? (e.g. Background processing, Audio, AirPlay, and Image in Image)
2. Which iOS API ought to I used to maintain a timer operating within the background?