How to submit float score as time to Game Center?

Okay, so I have Game Center up and running but when the game submits the time the player achieved at the end of the race, the leaderboard shows it as say 00:00:00:65, instead of 00:01:05:34. So it seems to be taking my float score and truncating it and not converting it to be used as time. I was foolishly hoping GC would handle that for me. So my question is, how do I submit a time score as a single float? I understand how to split a float into hours/minutes/seconds but seeing as GC only accepts one variable, that's not an option.

Any help gladly appreciated!