In the Julian Calendar (Old-Style Calendar used in all European countries before 1582 AD), leap years (366-days long) were considered all the years divisible by 4. The algorithm for the definition of a leap year in the Gregorian Calendar (currently used calendar) is a little more complicated, in order to diminish the number of the leap years, since the true duration of the tropical year is not 365+1/4 (365.25) days but a little smaller: 365.21249879 days. To this account it was decided that the hundred years (divisible by 100) should not be included in the leap years, except for the 4-hundred years (divisible by 400). According to this definition, years 1700, 1800, 1900, 2100 are considered as leap (366-days long) in the Julian Calendar but not leap (365-days long) in the Gregorian Calendar. On the other hand years 1600, 2000, 2400 are leap in both calendars. This rule explains the incremental deviation of the Julian (Old) Calendar from the more accurate Gregorian (New) Calendar, since after the new-year day of the hundred years except for the 4-hundred years, the Julian Calendar shifts one more day to the real dates of the Gregorian Calendar. Now the delay is 13 days but after 2100 it will become 14 days, after 2200 15 days and so on.
The most strange transition from the Julian to the Gregorian Calendar occurred in Sweden. In 1700, a year considered as leap in Julian Calendar but not leap in Gregorian, they decided that the day following February 28th would not be 29th (as was in the Julian Calendar) but March 1st. This resulted to a very strange situation. Swedish Calendar after March 1st of 1700 was 1 day in advance to the Julian calendar and 10 days behind the Gregorian Calendar. They decided to correct it and realign it to the Julian Calendar in February 29th of 1712, by inserting one more leap day, February 30th; thus 1712 in Sweden lasted 367 days. At last on February 17th of 1753, they decided that the next day would be March 1st, aligning their calendar to the Gregorian.