The name that a calendar gives to a day.
Even for one calendar (such as the Gregorian). The date may take different forms. Eg.
November 16, 2001 11/16/2001 - has been used in places with American influence
16 November 2001 16/11/2001 - has been used in places with European influence
2001 November 16 2001-11-16 - the best format to use now.
Please refer to the short list of the major users. This format is unambiguous and should now be the default way of working everywhere.