provides attributes for normalization of elements that contain datable events to a custom dating system (i.e. other than the Gregorian used by W3 and ISO). supplies the value of a date or time in some custom standard form.

The following are examples of custom date or time formats that are not valid ISO or W3C format normalizations, normalized to a different dating system

Alhazen died in Cairo on the 12th day of Jumada t-Tania, 430 AH .

The current world will end at the end of B'ak'tun 13.

The Battle of Meggidu (23rd year of reign of Thutmose III).

Esidorus bixit in pace annos LXX plus minus sub die XI mensis Octobris indictione IIII

Not all custom date formulations will have Gregorian equivalents.

The when-custom attribute and other custom dating are not contrained to a datatype by the TEI, but individual projects are recommended to regularize and document their dating formats.

 specifies the earliest possible date for the event in some custom standard form. specifies the latest possible date for the event in some custom standard form. indicates the starting point of the period in some custom standard form. The Great Fire of London

The Great Fire of London burned through a large part of the city of London.

 indicates the ending point of the period in some custom standard form. supplies a pointer to some location defining a named point in time with reference to which the datable item is understood to have occurred supplies a pointer to a calendar element or other means of interpreting the values of the custom dating attributes. Contayning the Originall, Antiquity, Increaſe, Moderne eſtate, and deſcription of that Citie, written in the yeare 1598. by Iohn Stow Citizen of London.

In this example, the calendar attribute points to a calendar element for the Julian calendar, specifying that the text content of the date element is a Julian date, and the datingMethod attribute also points to the Julian calendar to indicate that the content of the when-custom attribute value is Julian too.

 μηνὶ Ἰουνίου εἰς κ ἔτους ςωϞ

In this example, a date is given in a Mediaeval text measured "from the creation of the world", which is normalised (in when) to the Gregorian date, but is also normalized (in when-custom) to a machine-actionable, numeric version of the date from the Creation.

Note that the datingMethod attribute (unlike calendar defined in att.datable) defines the calendar or dating system to which the date described by the parent element is normalized (i.e. in the when-custom or other X-custom attributes), not the calendar of the original date in the element.