       (?:\s+|[-:Tt])  # separator before clock
      (\d\d?):?(\d\d)    # hour:min
      (?::?(\d\d(?:\.\d*)?))?  # optional seconds (and fractional)
   )?                    # optional clock
      \s*
   (?:
      ([-+]?\d\d?:?(:?\d\d)?
       |Z|z)             # timezone  (Z is "zero meridian", i.e. GMT)
      \s*
   )?$c