Self explanatory, Timer which counts up rather than down
The current timer only counts down.
As a workaround (sort of) there is a card in HACS called “Timer bar card”:
It can be configured so that the bar gets longer as the timer counts down. The displayed time still goes downwards, though.
Don’t forget to vote for your own FR! 
This has already been requested. You can vote and comment here: CountUP Timer