C.2. Time and Date format

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When possible, dates should be expressed in ISO 8601 and RFC 3339 compatible Calendar format, that is to say "YYYY-MM-DD", or otherwise a Combined Date and Time representation (<date>T<time>, so "YYYY-MM-DDThh:mm"). Separators for the date (hyphen) and time (colon) components are required to conform to both ISO 8601 "extended" format and RFC 3339 required format.

In the case of Date and Time representation, a timezone can be added as per RFC 3339 and the newer revisions of the ISO 8601 standard (which allow for negative offsets):

by appending the letter Z for UTC (e.g. "YYYY-MM-DDThh:mmZ"), or
by appending the complete "hours:minutes" positive or negative time offsets from UTC (e.g. "YYYY-MM-DDThh:mm+03:00").

For more details see examples at Wikipedia page on ISO 8601 and the publicly available RFC at RFC 3339.

Other representations from those specifications are not necessarily supported.