I've always considered it to be an error.
My understanding of the scene. Newman... like many people during that time... believed that the new millennium would begin on Jan 1, 2000. However, this is not true. The year 2000 was the last year of the previous millennium, so to celebrate a new millennium, you'd want your party on Dec 31, 2000... to ring in Jan 1, 2001. Newman has booked the wrong date. His party is a year too soon.... thus Jerry's statement that it's "late" doesn't make any sense.
Despite seven years later.. if anyone's still confused.. here's the way I was able to explain to many people. Imagine you're in a summer baseball league that has seasons which are five weeks long. So... week 1 is the first week... week 2 is the second..etc..etc. On the sunday of week 5, you're not done with the season yet. You have to finish that week too to complete the entire season. If you wanted to celebrate the end of the season.. you'd do it on saturday of week 5.
So.. now think about years. A millennium is 1000 years. Year 1 is the first... year 2 is the second..etc..etc. (like Jerry said... there is no year 0). So... on Jan 1, 2000... you are entering the final year of the current millennium (like the final week of the baseball season). To celebrate the end of the millennium and ring in the next one, your party would be the end of 2000. The switch from 00-01 was the right date... (and I hadgreat time that night :-)
Hope that helps anyone still confused.