Create a minimal string compatible with ISO-8601. Possible output formats
are (for example): 2012, 2012-06, 2012-06-23, 2012-06-23T23:30Z,
2012-06-23T23:30+0100, 2012-06-23T23:30:59Z, 2012-06-23T23:30:59+0100
a newly allocated string formatted according
to ISO 8601 and only including the datetime fields that are
valid, or NULL in case there was an error. The string should
be freed with g_free().
Create a minimal string compatible with ISO-8601. Possible output formats are (for example): 2012, 2012-06, 2012-06-23, 2012-06-23T23:30Z, 2012-06-23T23:30+0100, 2012-06-23T23:30:59Z, 2012-06-23T23:30:59+0100