If you want the answer to be in minutes & seconds, use [m]:ss as the format
for the answer cell. If you want hours, minutes, & seconds, use [h]:mm:ss
It doesn't matter which of the Excel time formats you use for display in the
input cells, as format affects only the display, not the underlying value.
Remember when you are inputting your data that 12:34 will be interpreted as
hh:mm, not mm:ss
You'll need 0:12:34 or 12:34.0 to get it to recognise the time as mm:ss