formatIso8601
Format @datetime in ISO 8601 format, including the date, time and time zone, and return that as a UTF-8 encoded string.
Since GLib 2.66, this will output to sub-second precision if needed.
Return
a newly allocated string formatted in ISO 8601 format or null in the case that there was an error. The string should be freed with g_free().
Since
2.62