=TEXT(B2-A2, "h:mm:ss")

=A2-B2	Calculates the difference between the time values in cells A2 and B2.
=TIMEVALUE("8:30 PM") - TIMEVALUE("6:40 AM")	Calculates the difference between the specified times.
=TIME(HOUR(A2), MINUTE(A2), SECOND(A2)) - TIME(HOUR(B2), MINUTE(B2), SECOND(B2))	Calculates the time difference between values in cells A2 and B2 ignoring the date difference, when the cells contain both the date and time values.