Well if you add 1000 days to 00/00/1900 you get 26/09/1902, so it's got something to do with that.........what that is.......your guess is as good as mine.
It was written by Henry VIII to calculate when he figured he'd have enough of Anne Boleyn - alas, he hadn't realized that Excel dates started in 01/01/1900, and not in 1536 ...
Seriously, I think it's just a placeholder - stick any value in there, to see if it will work, and I'll return later and expand on it ...