Gregorian calendar
To make sure that the mistake did not happen again, they revised the 'leap year rule'. Until then, every fourth year had February 29, without exception. The change was that there would be no February 29 for every year that ends in 00 - unless it could be divided by 400. So the year 2000 was a leap year, because it could be divided by 400, but 1700, 1800, and 1900 would be common years, with no February 29.