It's hardly a mistake just acknowledging that the time passing on Alpha might not be the same as elsewhere. If the moon is travelling fast then it might only be 800 days on Alpha, yet centuries into the future.
Problem is that, obviously, when the Moon enters a solar system it must slow down, as they're around for days not hours, and the Eagles have enough delta-very to make a landing (an argument for the Eagles being fitted with the gravity device Bergmann creates in Black Sun, to allow them to decelerate more than is realistic, and to create a forcefield heat shield for them).