QDM v5.3 Annotated and QDM v5.4 have the exact same examples in Appendix A.3.4 regarding cumulative medication duration. 

The following section describes one provided example for duration calculation.

Ibuprofen 800 mg every 12 hours , dispense 14 written at 17:00 on August 7 would
include start dateTime as 201708071700 and the end dateTime as 20170814. For this
example, the calculated result is August 14 minus August 7 = 7 days.

We believe that the difference in the provided example would be 7 days; however, It is unclear to us whether the uncertainty in the end dateTime should result in a duration interval of [6,7] or as it's currently stated as 7 days.