Date Header is Required
Orig-date or Resent-date field
Just what it looks like:
Date: Sat, 22 May 1993 05:46:55 +0000 (GMT)
The only optional parts of the date specification are the day of the week and the seconds.
Timezone may be given in the usual ways: EST, EDT, etc, UT, GMT, even military (Z,A,B,M,N).
Timezone is preferred as a numeric offset from GMT