update date-time format so that it is more conformant with RFC 3339

based on the grammar and note here: http://tools.ietf.org/html/rfc3339#section-5.6

- the existing regex does not allow for a time offset
- the existing regex does not allow for a lowercase 't' or lowercase 'z'
- the existing regex does not allow for a space character instead of a
  'T', as described in the note
- the existing regex does not allow arbitrarily many digits in the
  second fraction part