I think it demonstrates precession, and the difference between the tropical and sidereal years.

Of course, the times were actually calculated from the known rate of precession; we couldn't exactly have said that the summer solstice occurred at 22:00 hours on 2nd July, 1000 BCE. There, you know, wasn't even a July then, given that it was named after a dude who wouldn't be born for another 900 years.

EDIT: As mentioned by other comments, it also captures the shift between the Julian and Gregorian calendars.