In Excel, times are decimal fractions of a day that are simply formatted
to look like time. Thus =2.5/24/60 when formatted as time displays as
0:02:30 (2 minutes and 30 seconds).
However I can't figure out what what you are trying to do in your
example. What is the .5 in 1.5 supposed to represent? If it is one and
a half minutes, then 1.5+1.5 is 3 minutes, not 2 minutes and 30 seconds.