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
Return: 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
Return: 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().