Custom format to display numbers as duration

Hi.
I'm trying to calculate a sum for over 1000 integers in a worksheet that are in an hour: minute: second format, but not time of day. They are in duration (length). Every time I calculate it, Excel displays the total in time of day format (AM / PM). I want to know the TOTAL number of hours: minutes: seconds of all my entries but I do not see a selection for this in any of the custom formats.

If you have any insight to how to work around this or how to create a custom format, I would be very happy to get your advice. Thanks!

Patrick

Wow- thank you. That was fast! And very helpful.
Patrick