Yes, I agree with the Nr Fairy - at the moment Excel thinks you mean the time of day.
I know that the [h]:mm:ss format makes Excel rollover the hours instead of treating it as the time of day. Using Format, Cells, Number Tab, choose Custom and scroll down to near the bottom, where the [h]:mm:ss format is.
I've never tried [hh]:mm. Also, don't forget to add a 0 (zero) if your time is less than 1 hour.