The image config's created property is defined as anything compatible with ISO-8601, but this should be restricted to date/time formats compatible with section 5.6 of RFC-3339.

The RFC-3339 format is compatible with, but stricter than, ISO-8601 format. It removes much ambiguity and undesired variability, such as 2-digit years or the time "24:00". It is also available freely on the Internet, unlike the ISO standard.