Currently, the PTO calendar works on hourly increments, however, the delegation option works in full-day increments only.
If someone is out on "Friday morning" the only way they could delegate approvals for Friday is to set it up for the full day and turn it off when they arrive.
If they're off in the afternoon the only way to delegate is to way until they're ready to leave & delegate approvals.
It would make sense that if you can enter PTO in partial days (hourly, even) that you could set up your delegation to match in time increments.