date and UTC and GMT odd behaviour

[Mike 'Mike' Jones]

I have just noticed an anomaly with the UTC and GMT time zones in the 
standard coreutils.  Around the Unix epoch they are not delivering the 
same times:

--At midnight:
$ date -d "Tue Jan 01 00:00:00 UTC 1970" +%s
0
$ date -d "Tue Jan 01 00:00:00 GMT 1970" +%s
-3600

--At 1am
$ date -d "Tue Jan 01 01:00:00 UTC 1970" +%s
3600
$ date -d "Tue Jan 01 01:00:00 GMT 1970" +%s
0

A little bit of trial-and-error and I find that the anomaly is limited to
between 6pm, 23 Jun 1968 GMT and 2am, 31 Oct 1971:

$ date -d "Sat Oct 31 01:59:59 UTC 1971" +%s
57722399
$ date -d "Sat Oct 31 01:59:59 GMT 1971" +%s
57718799

$ date -d "Sat Oct 31 02:00:00 UTC 1971" +%s
57722400
$ date -d "Sat Oct 31 02:00:00 GMT 1971" +%s
57722400

and

$ date -d "Sun Jun 23 18:00:00 UTC 1968" +%s
-48060000
$ date -d "Sun Jun 23 18:00:00 GMT 1968" +%s
-48063600

$ date -d "Sun Jun 23 17:59:59 UTC 1968" +%s
-48060001
$ date -d "Sun Jun 23 17:59:59 GMT 1968" +%s
-48060001


This means that using:
$ date -d "Tue Jan 01 00:00:00 GMT 1970 + NNNNN seconds"
returns dates that are one hour early.  This affects scripts that 
I'm writing which use credential lifetimes passed as strings encoded as 
seconds since the Unix epoch.


$ date --version
date (coreutils) 4.5.3
I get the same result with date versions 5.0 and 5.2.1.

Cheers,
Mike

--
http://www.sve.man.ac.uk/General/Staff/jonesM/

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