For level 1 it's simple, use the time that displays after touching "the portal"

For Levels 2-27 it's trickier tho

Let´s say you want the time for level 15, then you take the timer for level 15 let's say 100 seconds - the timer for level 14 let's say 70 seconds

100-70=30, level 15 took 30 seconds for you to finish