The values of dates used by the properties, events and methods of the Calendar object are specified using IDNs (International Day Numbers).

An IDN is an integer that represents a date as the number of days starting at Jan 1^{st} 1900, that is, Jan 01 1900 is IDN 1.

Note that the value of IDNs calculated by Dyalog APL are one less than IDNs calculated by Microsoft Excel which incorrectly assumes that 1900 was a leap year.