Of course. There is no time such time as 15:00 but there is a time of 15
minutes past midnight [12:15:00 am]. Using your format of mm:ss format, this
will show the 15 minutes you asked about vs the 15 hours (ie 3 pm) you had
been getting.
If you are looking for 15 mnute increments...
I suggest that you enter .25 in one cell and convert it to minutes in
another cell.
For example:
Cell A1 = .25
Cell B1 =A1*60
This will give you the NUMBER 15 vs the TIME value of 15 minutes
If you are trying to do something like a timecard...
Start time - A1 = 08:00 AM
End time - B1 = 04:30 PM
Break in hrs - C1 = 0.50
Paid Time - D1 =((+B1-A1)*24)-C1
If you are trying to subtract one time from another...
A1 = 00:15:25
B1 = 00:25:18
C1 =(B1-A1)*24*60*60 = 593 seconds
OR
C1 =(B1-A1)*24*60 = 9.883 minutes
OR
C1 =+B1-A1 = 00:09:53 <= formatted as 'mm:ss'
Hope one of these options is what you are looking for.
Sincerely,