Time fields are really number fields that count the number of seconds since 12 midnight.
If we're not talking about emergency rooms where the patients might come before midnight and leave after midnight...
Define two time fields TimeArrived and TimeSeen.
Define a calculation, WaitTime as
(TimeSeen - TimeArrived) / 3600
and it will compute the number of hours the patient waited.
If you prefer,
TimeSeen - TimeArrived
Set to return "Time" will return the elapsed time in seconds and you can use standard time formats like HHMMSS to display the result.