Click picture to ENLARGE.
For centurial years it is NOT enough to be divisible just by 4. It also has to be divisible by 400. This is was another major improvement to the Gregorian calendar to keep the year lengths as close to 365.242199 days as possible.
So, 1700, 1800, 1900 were NOT LEAP YEARS since they are NOT divisible by 400.
But, 2000 was, because it is divisible by 400.
Excel has 1900 as a LEAP YEAR.
Yes, I am probably the ONLY one in the whole world that would NOTICE and CARE.
But, I do a LOT of date span calculations — example when I am working out ICE-OUTS for lakes. In such calculations, being the pedant I am, I take into account Leap Years. Otherwise you are mistating some date spans.
That is how I noticed.
You can check it out.