1. A year is a leap year if the year is divisible by 4, UNLESS
2. If the year is divisible by 100, It is not a leap year, UNLESS
3. If the year is divisible by 400, It is a leap year.
So, the year 2000 (divisible by 400) was a leap year, but 2100 (divisible by 100, but not divisible by 400) will not be a leap year.
The change from Julian (Julius Caesar) calender to the Gregorian (Pope Gregory XIII) was a mess, and different countries changed at different times, starting in 1582 with Italy. It was not until 1929 that all countries of the world were using the same calender. Not only that, but the change to the Gregorian calendar also need to skip anywhere from 10 days (for the early adapters) to up to 13 days (for the late comers.)
Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752 was followed by Thursday, 14 September 1752.
Sweden had a particularly difficult time making the change.
Sweden's relationship with the Gregorian Calendar was a difficult one. Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually, by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. In the meantime, the Swedish calendar would be out of step with both the Julian calendar and the Gregorian calendar for 40 years; also, the difference would not be constant but would change every 4 years. This system had potential for confusion when working out the dates of Swedish events in this 40-year period. To add to the confusion, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar (according to the transition plan) should now have been 8 days behind the Gregorian, but was still in fact 10 days behind. King Charles XII recognised that the gradual change to the new system was not working, and he abandoned it.
However, rather than proceeding directly to the Gregorian calendar, it was decided to revert to the Julian calendar. This was achieved by introducing the unique date 30 February in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, 17 February was followed by Thursday, 1 March.