At the moment, you’ll have to accept it as a text entry and perform the necessary conversions and calculations yourself. I’ve put together a quick demonstration of some possible routines to do so here. The Duration field accepts a timestamp in either ‘HH:MM’ or ‘HH:MM:SS’ format; breaks out Hours, Minutes, and Seconds; converts Hours, Minutes, and Seconds to TotalSeconds; extracts GetHours, GetMinutes, and GetSeconds from TotalSeconds; and, finally, returns GetHours, GetMinutes, and GetSeconds to NewDuration in ‘HH:MM:SS’ format. (Alternate formulas are also provided that convert Duration directly to TotalSeconds2 and TotalSeconds directly to NewDuration2.)
The parsing routine will correctly interpret timestamps with either 1- or 2-digit unit sections, in any combination (e.g., ‘HH:MM:SS’, ‘H:MM’, ‘HH:M:SS’, ‘H:M:S’, and so on).
Note: These routines support only to 1-second granularity, but could easily be enhanced to support fractional-second or per-frame timing.
I realize this wasn’t what you were hoping to find, but I hope it’s an acceptable substitute…