I know the 5th birthday was on 21st May this year, but I found this page:

http://setiathome2.ssl.berkeley.edu/classpages/days/2451239.html

which has people who registered on Monday March 1st 1999.

Any explanations?

Or are these people just the programmers who got themselves set up on SETI
before it officially started? Which doesn't seem right as there are other
people who have registered before the 21st May 1999, ie here

http://setiathome2.ssl.berkeley.edu/classpages/days/2451275.html on 6th
April

Oh, and can anyone explain the numbering system at the end of the web
address for registration classes? The 2451*** up to 2453***, as far as I can
tell, assuming that the date directly corresponds to the number, which it
does appear to, then "1" would have been about 6,770 years ago. I'd like to
know the significance of the starting date, if any.

Curiously,

Nick