One way:

Use a custom number format (Format/Cells/Number/Custom, mm:ss.00).

Note that if you enter a time as 12:34 it will be interpreted as
hours:minutes. If you need to enter whole minutes/seconds, do it like

12:34.0




In article ,
Eileen wrote:

My children are swimmers. I am not able to use the available time formats to
work for their swimming times, the times are

mm:ss.fraction of seconds

I've put a spreadsheet to show their best times and then have columns that
list a variety of time standards. What I'm having trouble doing is the
column that shows the amount of time needed to drop to make the standard. It
should be a simple column subtraction. So far, I'm having to hand subtract
the when the minutes are long as in distance events, time standard is 5:31.78
and one swims a 6:18.92.