[LEAPSECS] Crunching Bulletin B numbers (POSIX time)

Tony Finch dot at dotat.at
Mon Feb 21 09:57:44 EST 2011

On Mon, 21 Feb 2011, Ian Batten wrote:


> We're casually assuming that everyone understands GMT to be "sort of"

> UTC, and not defining our terms any more accurately than that...

Actually in the context of email timestamps GMT is fairly precisely
specified to be the same as a zero offset from UTC, though the wording
in the specification has some warts due to its very long history.
(Search for "zone" in RFCs 561, 680, 724, 733, 822, 2822, 5322.)

Even so, Sendmail is not following the specification correctly. "GMT" as a
time zone is "obsolete syntax" which must be recognized by a parser (for
compatibility with old software) but must not be emitted.

RFC 5321 section 4.4 "Trace Information":

Time-stamp-line = "Received:" FWS Stamp <CRLF>

Stamp = From-domain By-domain Opt-info [CFWS] ";"
FWS date-time
; where "date-time" is as defined in RFC 5322 [4]
; but the "obs-" forms, especially two-digit
; years, are prohibited in SMTP and MUST NOT be used.

RFC 5322 section 3.3 "Date and Time Specification":

date-time = [ day-of-week "," ] date time [CFWS]

time = time-of-day zone

zone = (FWS ( "+" / "-" ) 4DIGIT) / obs-zone

The zone specifies the offset from Coordinated Universal Time (UTC,
formerly referred to as "Greenwich Mean Time") that the date and
time-of-day represent.

(I think "formerly" here is the result of an editorial mistake. Previous
versions of the specification referred to "Universal Time" and the update
to UTC was botched.)

RFC 5322 section 4.3 "Obsolete Date and Time":

obs-zone = "UT" / "GMT" / ; Universal Time
; ...

In the obsolete time zone, "UT" and "GMT" are indications of
"Universal Time" and "Greenwich Mean Time", respectively, and are
both semantically identical to "+0000".

f.anthony.n.finch <dot at dotat.at> http://dotat.at/
Plymouth: Southeasterly becoming cyclonic 5 to 7, then northerly backing
southerly 3 or 4. Rough or very rough. Rain. Moderate or poor.

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