you can use the Time() function to change the text to a time.
timevar t:= time({your elapsed time field});
this formula will change the difference into a true time...i.e. 02:30:00 becomes two thirty in the morning. however that doesn't matter as the rest of the formula converts this into elapsed seconds which is the most important piece.
in fact you can also change the Display String formula to something like this since if you do wish to display elapsed time...this assumes that no one's waiting in the waiting room for more than 24 hours though.
numbervar ts:= currentfieldvalue;
totext(time(dateadd("s",ts,currentdate)),'HH:mm:ss')