Robert Ancell authored Aug 25, 2016 and Philip Withnall committed Sep 11, 2017

This supports a subset of ISO 8601 since that is a commonly used standard for
storing date and time information. We support only ISO 8601 strings that contain
full date and time information as this would otherwise not map to GDateTime.
This subset includes all of RFC 3339 which is commonly used on the Internet and
the week and ordinal day formats as these are supported in the GDateTime APIs.

(Minor modification by Philip Withnall to change API versions from 2.54
to 2.56.)

https://bugzilla.gnome.org/show_bug.cgi?id=753459