I think I understand what you want to do....

You would prefer to enter times (hours and minutes) with a decimal separator, instead of a colon.

So 15.45 would mean 3:45 PM

If that's true, try something like this:
A1: start time....eg 11 (meaning 11 AM)
B1: end time.....eg 15.45 (meaning 3:45 PM)

This formula calculates the difference, in hours and minutes, between those 2 times
C1: =MOD(DOLLARDE(B1,60)-DOLLARDE(A1,60),24)/24
Format that cell as time (hrs:mm)

In the above example, the formula returns: 4:45

Here are some other examples: 
Start       End         Elapsed
 9.00       11.00       2:00
23.00        2.30       3:30
16.00       19.25       3:25
Is that something you can work with?