In cell A1 i have a time 21:00

In cell B1 I have a length of time ie 00:20 ( 20 minutes not 21 mins past midnight )

In Cell C1 I have a number ie 50.

I need a formula to look in cell A1 sunbtract B1 so in the above case would = 21:00 - 00:20 so would give 20:40. The if that time is between 19:00 and 21:00 return the value in C1 otherwise return a zero value.

Any help appreciated.