Easy to do with module time.
Enter the two times in hh:mm:ss format, add a date and use time.strptime() and the proper format string to create two time tuples.
time.mktime() converts each time tuple to seconds since epoch. Now you can take the time difference in seconds and give the result in seconds or minutes or fractional hours.
# time difference between two (12 hour) times of a day
import time
hms1 = "08:15:00AM"
hms2 = "04:45:30PM"
mdy1 = "05/15/06 "
epoch_seconds1 = time.mktime(time.strptime(mdy1+hms1, "%m/%d/%y %I:%M:%S%p"))
epoch_seconds2 = time.mktime(time.strptime(mdy1+hms2, "%m/%d/%y %I:%M:%S%p"))
time_diff = epoch_seconds2 - epoch_seconds1
print time_diff, "seconds"
print time_diff/60, "minutes"
print time_diff/(60*60), "hours"