This is not surprising behaviour. If you subtract 12 from 10, you get
-2 every time.
If you want your result to be positive, make it the first parameter
and subtract the smaller (earlier) time from it. If your result is
negative, then you know you have crossed into another day (13:00:00 -
17:00:00 = -04:00:00, meaning the 17:00:00 was from yesterday
afternoon). If your times can be more than 24 hours apart, then
working with SUBTIME won't help you much, by itself.
Reference:
http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html
SUBTIME(expr,expr2)
SUBTIME() subtracts expr2 from expr and returns the result. expr is a
time or datetime expression, and expr2 is a time expression.
