How are you trying to calculate difference?

Is the Appt Time should be taken as for the same date as one in the column which contains date and time?

What is really your wait time?

Is this any help to you:

DECLARE @dt DATETIME, @appttime CHAR(4)

SELECT @dt = GETDATE(), @appttime = '0730'

SELECT @dt

,@appttime

,DATEADD(MINUTE, CAST(RIGHT(@appttime,2) AS INT)

,DATEADD(HOUR, CAST(LEFT(@appttime,2) AS INT), @dt))