aus der Dokumentation der Cocoa class, die den Kalender steuert:

How NSCalendar Models the Gregorian Calendar
The Gregorian calendar was first introduced in 1582, as a replacement for the Julian Calendar. According to the Julian calendar, a leap day is added to February for any year with a number divisible by 4, which results in an annual disparity of 11 minutes, or 1 day every 128 years. The Gregorian calendar revised the rules for leap day calculation, by skipping the leap day for any year with a number divisible by 100, unless that year number is also divisible by 400, resulting in an annual disparity of only 26 seconds, or 1 day every 3323 years.

To transition from the Julian calendar to the Gregorian calendar, 10 days were dropped from the Gregorian calendar (October 5–14).

After the Gregorian calendar was introduced, many countries continued to use the Julian calendar, with Turkey being the last country to adopt the Gregorian calendar, in 1926. As a result of the staggered adoption, the transition period for countries at the time of adoption have different start dates and and a different number of skipped days to account for the additional disparity from leap day calculations.

NSCalendar models the behavior of a proleptic Gregorian calendar (as defined by ISO 8601:2004), which extends the Gregorian calendar backward in time from the date of its introduction. This behavior should be taken into account when working with dates created before the transition period of the affected locales.