A quick and dirty crosswalk of an unknown date(time) input to EML that really
only works for ISO8601 input. All other formats will fail and be returned
as-is as a calendarDate. From there the user will need to do a conversion
themselves.
crosswalk_datetime(input)(list) A list with members calendarDate and time. time will
be NULL if parsing fails or if the time string inside input isn't
ISO8601
(character) Some unknown date(time) input