--- Time#xmlschema([fractional_seconds])
--- Time#iso8601([fractional_seconds])
    XML SchemaƤdateTimeȤ
    ɽʲηʸ֤ޤ:

      CCYY-MM-DDThh:mm:ssTZD
      CCYY-MM-DDThh:mm:ss.sssTZD

     TZD  Z ޤ [+-]hh:mm Ǥ

    If self is a UTC time, Z is used as TZD.
    [+-]hh:mm is used otherwise.

    fractional_secondsϾʲäꤷޤ
    fractional_secondsΥǥեͤ0Ǥ

