calculate time difference

is it possible to calculate the time difference between 2 cells? for example, if cell a2 is the start time and cell b2 is the finish time, what would the formula be to calculate the time difference.

a2 time is 14:00:00
b2 time is 15:30:00

i would like for c2 to display the time in the same format of hours:minutes:seconds. in excel, i would enter =b2-a2 in cell c2. i tried this but i got an error message…”!error! b2 does not contain a number or expression”

thanks for your help.