How long should my microwave run?

Python 2, 19 bytes

lambda t:t-t/100*40

Try it online!

Japt, 6 bytes

ìL ì60

Test it online! ìL converts to base-100, and ì60 converts back to base 60, resulting in floor(n/100)*60 + n%100. Also works with hours (10000 -> 3600, the number of seconds in an hour).

Pyth - 9 8 bytes

Converts input to base 100, then interprets that as a base 60 number.

ijQ*TT60

Test Suite.