Calendar

is an option for calendar functions which specifies which calendar system to use.

With Automatic, the Julian calendar is used before September 3, 1752, and the Gregorian calendar after September 13, 1752.

Compute the number of days in the Islamic year 1420:

Compute the number of days in the Jewish year 5760:

In the Julian calendar, the year 1900 was a leap year:

This is not true in the standard (Gregorian) Western calendar:

Needs["Calendar`"]

Compute the number of days in the Islamic year 1420:

Out[2]=

Needs["Calendar`"]

Compute the number of days in the Jewish year 5760:

Out[2]=

Needs["Calendar`"]

In the Julian calendar, the year 1900 was a leap year:

Out[2]=

This is not true in the standard (Gregorian) Western calendar:

Out[3]=