Calculate time difference from sensor


It feels like this is possible somehow, but i just can’t figure out how…
I have a sensor that gives me a time, for example 15:00. I wan’t to make an other sensor that calculates the time difference between now and 15:00, like a countdown timer for that value, and when 15:00 is met, the first sensor gets a new value, like 15:20, then the countdown sensor will re-count down to 15:20.

Anyone has any ideas?