user variable as a timer (count down)

[voyo]

Hi,
I have a suggestion, what if we can have a possibility to create user variable of counter type, which can count down a set time.
Example usage -
create such variable, name it like 'presence_in_room', set it to default 3minutes, then allow system to count down every second.
When it count to 0 - system consider as there is no presence in that room. But there are other devices , which can reset the timer, or add + few minutes. (PIR, input from camera detecting movement, light switch , anything).

Or... maybe there is other way to achieve this functionality ??
(I tried to achieve something similar using events, but problem is that it can decrease value only in 1minute intervals, which seems is too low)