Problem with SUM Function with Military Time

I have a column of trip times that were calculated utilizing military time and the custom format of [h]:mm. Doing this results in trip times that calculate out perfectly. What I am not able to do, is total these trip times! How should my function read, and what formatting do I need to sum these trip time totals?

Ie. Trip Times Totals in Column:
6:00 (6 hours)
4:15 (4 hours, 15 minutes)
2:30 (2 hours, 30 minutes)

Should = 12 hours 45 minutes... instead, the total still reads 0

What am I doing wrong?

You would think this would be the formula... but the total is 0 with this formula. Is this because my custom format is [h]:mm? I used this formatting option to make the trip times calculate properly.

Screen Shot.JPG

The sum works, but the time calculation does not show in the cell... Screen Shot - .JPG

Resolved it...

Formula for calculating trip time: =MOD((A2-A1),1) <--- formula for time over 24 hour period with cell formatted as: [h]:mm

Once I made this change, trip calculation and column sum are calculating correctly. Thank you for your help.