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http://asg.web.cmu.edu/rfc/rfc822.html#sec-5

5.1 SYNTAX

     date-time   =  [ day "," ] date time        ; dd mm yy
                                                 ;  hh:mm:ss zzz

     day         =  "Mon"  / "Tue" /  "Wed"  / "Thu"
                 /  "Fri"  / "Sat" /  "Sun"

     date        =  1*2DIGIT month 2DIGIT        ; day month year
                                                 ;  e.g. 20 Jun 82

     month       =  "Jan"  /  "Feb" /  "Mar"  /  "Apr"
                 /  "May"  /  "Jun" /  "Jul"  /  "Aug"
                 /  "Sep"  /  "Oct" /  "Nov"  /  "Dec"

     time        =  hour zone                    ; ANSI and Military

     hour        =  2DIGIT ":" 2DIGIT [":" 2DIGIT]
                                                 ; 00:00:00 - 23:59:59

     zone        =  "UT"  / "GMT"                ; Universal Time
                                                 ; North American : UT
                 /  "EST" / "EDT"                ;  Eastern:  - 5/ - 4
                 /  "CST" / "CDT"                ;  Central:  - 6/ - 5
                 /  "MST" / "MDT"                ;  Mountain: - 7/ - 6
                 /  "PST" / "PDT"                ;  Pacific:  - 8/ - 7
                 /  1ALPHA                       ; Military: Z = UT;
                                                 ;  A:-1; (J not used)
                                                 ;  M:-12; N:+1; Y:+12
                 / ( ("+" / "-") 4DIGIT )        ; Local differential
                                                 ;  hours+min. (HHMM)

5.2 SEMANTICS

    If included, day-of-week must be the day implied by the date specification.

    Time zone may be indicated in several ways. "UT" is Univer- sal Time
    (formerly called "Greenwich Mean Time"); "GMT" is per- mitted as a reference
    to Universal Time. The military standard uses a single character for each
    zone. "Z" is Universal Time. "A" indicates one hour earlier, and "M"
    indicates 12 hours ear- lier; "N" is one hour later, and "Y" is 12 hours
    later. The letter "J" is not used. The other remaining two forms are taken
    from ANSI standard X3.51-1975. One allows explicit indication of the amount
    of offset from UT; the other uses common 3-character strings for indicating
    time zones in North America.