The note on the calendar says “pick up Derek at 6 am for airport.” Not FROM airport.
Without knowing more, we cannot know if that means Derek was departing by air, arriving by air, or picking something up at the airport.
Given the early morning hour, I think it more likely RH was giving Derek a ride to the airport for an early morning departing flight. And given that, I think it likely Derek lives in the area, temporarily flew out, but had returned sometime in the intervening week or so to be at RH’s for the cutting of the post (if in fact Derek did that and the notation was not simply to provide cover to the sound of RH using the band saw to cut something substantial).
Perhaps RH picked Derek up at the airport when Derek returned and didn’t put it in his calendar, or perhaps Derek found another means of getting home from the airport when he returned.