Working out Time difference

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I have the below that I am trying to work out.

People have 15:00 minute breaks at present. I have data of how long people have taken. So want to know the difference in time against 15 minutes.

Person 1 - 9:53
Person 2 - 20:24
Person 3 - 15:00
Person 4 - 12:30

And so on. At present I get ##### for when someone is below 15:00.

Please help as driving me nuts

 

[JustynaMK]

If you go to File > Options > Advanced and check "Use 1904 date system" then all negative time values should be now displayed correctly:

Otherwise you'd need to use some sort of conditional formula, like:
=IF(A1>TIME(0,15,0),TEXT(ABS(TIME(0,15,0)-A1),"-hh:mm:ss"),TIME(0,15,0)-A1)

Hope it works for you.