You can specify an interval as start/duration, start/end, or duration/end. So you could specify a negative offset as a duration and the end time. P2H/2021-02-

You can specify an interval as start/duration, start/end, or duration/end. So you could specify a negative offset as a duration and the end time. P2H/2021-02-26T13:30Z is two hours prior or T11:30Z on the same date.