The Timer tab can be used to alert you with an audible signal when a set amount of time has passed.
Open the Clock app, then select the Timer tab.
Set the time on the digital timer. Tap the Hour, Minute, and/or Second field(s) then enter the numbers as necessary.
If you don't need to change the Timer settings, just tap the green Start button on the time keypad to start the timer immediately.
Tap the Menu icon (at the upper-right corner of the screen) > Settings to set Alarm with vibration, Alert sound, and Alert volume for the timer.
Tap the green Start button (at the bottom of the screen) to begin the time countdown.
Once the alarm sounds, tap the red Stop button (at the bottom of the screen) to stop the timer.