As this game doesn't have the option to measure time for each level individually, we have to calculate it ourselves. (Doesn't apply for level 1, though.) You can use this tool:

https://datetimecalculator.net/subtract-time-calculator

You need to subtract the time you died or completed the previous level from the time you received after crossing the portal. Example:

6.266 - 3.300 = 2.966

EDIT: Necessary to point that out, if after subtracting times the result you got will be (.X17ms; .X34ms; .X67ms, .X84ms) it will be retimed to (.X16ms; .X33ms; .X66ms; .X83ms). It's because the number of frames is the same and there won't be 1ms difference that player can't do anything with.