Here is the reason explained by Joel himself: My First BillG Review
Basic uses December 31, 1899 as the epoch instead of January 1, 1900,
but for some reason, today's date was the same in Excel as it was in
I went to find an Excel developer who was old enough to remember why.
Ed Fries seemed to know the answer.
"Oh," he told me. "Check out February 28th, 1900."
"It's 59," I said.
"Now try March 1st."
"What happened to 60?" Ed asked.
"February 29th. 1900 was a leap year! It's divisible by 4!"
"Good guess, but no cigar," Ed said, and left me wondering for a
Oops. I did some research. Years that are divisible by 100 are not
leap years, unless they're also divisible by 400.
1900 wasn't a leap year.
"It's a bug in Excel!" I exclaimed.
"Well, not really," said Ed. "We had to do it that way because we need
to be able to import Lotus 123 worksheets."
"So, it's a bug in Lotus 123?"
"Yeah, but probably an intentional one. Lotus had to fit in 640K.
That's not a lot of memory. If you ignore 1900, you can figure out if
a given year is a leap year just by looking to see if the rightmost
two bits are zero. That's really fast and easy. The Lotus guys
probably figured it didn't matter to be wrong for those two months way
in the past.