Re: Sweet memories...
2000 was a leap year. 1900 and 2100 are not.
Most computers at the time assumed any year divisible by 4 is a leap year, when in fact years divisible by 100, but not divisible by 400 are not leap years.
Excel still thinks 29th February 1900 is a valid date, and most other spreadshhets copy this bug for compatibility.