The first thing that comes to mind is to use an array. For each object that has that timer running, push timer.duration-timer.currenttime, and the object's uid into an array. Sort the array to get his of the object with the lowest duration remaining.
If they all had the same duration to begin with you can just push and remove the object uid directly into an array, first in first out.
Or are they all on the same object? If you elaborated on your set up and how you are using the timers there may be better approaches. For finer control over timers you would normally use different tags.
EDIT: Below is a better idea.