The other option would be to manipulate the times using the substring-before() and substring-after() functions to make your own integer representations of the times that are entered
IE:
int(substring-before(#form/timestart, ":")) * 60 + int(substring-after(#form/timestart, ":"))
would in theory produce a number of minutes that could be subtracted from a similar calculation for a "time of end", but that wouldn't properly calculate for times that 'wrap around' (IE: 11:30pm -> 1:30am) or other edge cases.
-Clayton