Will both of the times always be within the same day? If so, you can use this formula to get the number of minutes between two times:
60 * (substring(laterTime, 12, 2) - substring(earlierTime, 12, 2)) + (substring(laterTime, 15, 2) - substring(earlierTime, 15, 2)) + (substring(laterTime, 18, 2) - substring(earlierTime, 18, 2)) div 60
Is that sufficient for what you're looking for?