The simplest way i can think to do this is to make a text layer which will be the timer text. And a Null layer which will have a Slider Control effect added to it. Then Copy and paste this expression into the source text property of the timer text layer:
Math.floor(thisComp.layer("Null 1").effect("Slider Control")("Slider"))
Then it’s as easy as animating the slider to whatever values you want to display. So make a keyframe with the slider at 30 at frame 0, and then 30 seconds later keyframe the slider to have a value of 0. This will have the text count down from 30-0 over 30 seconds. then 10 seconds later you can just set another keyframe for the slider with a value of 10, and the text will count back up from 0 to 10 over ten seconds.
If you want the slider to display”00:30″ instead of just “30”, the easiest way is to just make another text layer that contains”00:” and place it in front of the text that you’re animating with the slider.
And yes you can reuse that over and over in your future videos. If you want them all to display the same intervals you can just render out that comp with alpha and use it as a video layer that you’d place over future videos.