I think Mindfang's journal dating is an error on Hussie's part. As I said, a perigee is a point in a moon's orbit around the planet; you can't have several of those within a few days unless both their moons move at about the speed of sound. Also, Friendsim is quite definitely set BEFORE Joey turns up; the trolls in Friendsim have never seen a human before, none of them know more than a couple of the others yet, and the rebellion apparently hasn't begun yet.

As I said, AU where Joey just isn’t around, and the Rebellion is an underground group, for all we know it may not be well known among any of the Trolls in the Troll Call.

It could’ve been retconned so that Perigees are months now, or that maybe Bilunar Perigees and Perigees are actually two different measures of time, so honestly who knows. If we go by Homestuck Canon, they’re defined as days.