(TR-END-DURATION 'tr) denotes a duration of a time range tr.
(Tp= (Tp+ (Time-Range.Start-Time ?Time-Range) (Time-Range.Duration ?Time-Range)) (Time-Range.End-Time ?Time-Range)) (=> (Time-Range ?Time-Range) (Tp= (Tp+ (Time-Range.Start-Time ?Time-Range) (Time-Range.Duration ?Time-Range)) (Time-Range.End-Time ?Time-Range))) (=> (Time-Range.Duration $X $Y) (Duration $Y)) (=> (Time-Range.Duration $X $Y) (Time-Range $X))