|Not sure how you have your sheet set up, but try this:|
With your Start Time in A1 and your End Time in B1
1) 09:15:00 02:00:00
In cell C1 enter the formula:
Since B1 is the next day, you need to add 1 to get the correct time.
Don't forget to format cell C1 as Time
For a very good in depth explanation of how to do TIME Math in Excel see here: