Okay, the Bible does NOT say that Jesus Christ is coming back in 2000 (at least not in my Bible). In fact, Jesus tells his followers that nobody
knows the exact date of his return, and his followers are to be watching and waiting (and also getting their work done
The years are marked by our planet going around the sun. The calendars were created to help us keep track of that revolution - not to determine the
regularity of it.
It's the calendars that have been re-adjusted to make sure that they are accurate with the actual revolution of our planet around our sun. We not
only keep track of the motion of our planet around our sun, but we have people keeping track of how our planet also lines up with the constellations
Now it you really want to get technical, our planet does have a wobble on its axis such that over a long period of years, the summers and winters
would not line up with the same constellations, but that is a very long drawn out process that is not that significant in our short timeframe
perspective of counting years.