EU BST-->GMT issue

Moore, Martin martin.moore at
Thu Aug 30 15:09:31 UTC 2007

This is a consequence of the fact that the times from 01:00:00 to 01:00:59 occur *twice* on the day that summer time reverts to standard time.  The hours of the day starting from midnight look like this:

00:00 - 00:00:59 BST
01:00 - 01:59:59 BST
01:00 - 01:59:59 GMT
02:00 - 02:59:59 GMT

For the ambiguous times (01:00 - 01:59:59), the date command has to use one or the other instance of that time (either before or after the reversion); it happens to use the one after the reversion.  So there are three possible cases when you use the date command on this day:

1. From 00:00 - 00:00:59 -- Uses the time before the reversion (BST), the only reasonable choice.

2. 02:00 and later -- Uses the time after the reversion (GMT), the only reasonably choice.

3. 01:00 - 01:59:59 -- An ambiguous time, could be either BST or GMT; date happens to use the second one, GMT.

As a consequence of all this, you simply can't have the date command simply set the time into the interval 01:00 - 01:59:59 BST.  If you want to test the reversion, there are two ways that I can see to do it:

1. Set the time to 00:59 (BST), wait one hour plus one minute to observe the reversion.

2. Go through another timezone to get the time you want:

A) Set the timezone to GMT.
B) Use the date command to set the time to 00:59 (which is 01:59 BST).
C) Set the timezone back to London.
D) Type 'date' to verify that the time now shows as 01:59 BST.
E) Wait one minute to verify the reversion.

Martin Moore

-----Original Message-----
From: Dafydd Rhys-Jones [mailto:D.Rhys-Jones at] 
Sent: Thursday, August 30, 2007 11:00 AM
To: tz at; tz at
Subject: RE: EU BST-->GMT issue

If I manually set the time to 1 second after the reversion, it will set it to GMT, however, if I set it before the reversion time, and let it roll over said reversion time, it remains BST.

The first command below shows where I set it before rolling over reversion time, and the command at the end after all the date checks showing reversion did not occur is the command where I set it +1 second over reversion time, and it does set it to GMT.


Dafydd Rhys-Jones  |  Software Test Engineer
F5 Networks
  P 206.272.5555
 F 206.272.5556
  D 206.272.6280
M 206.335.1096


-----Original Message-----
From: Andy Lipscomb [mailto:AndyLipscomb at] 
Sent: Thursday, August 30, 2007 7:52 AM
To: tz at
Subject: RE: EU BST-->GMT issue

That looks correct to me. The reversion occurs at 0100 UTC (which is to say 2am BST or 1am GMT), so wallclock times from 1am until 2am are repeated at the fall-back.

J Andrew Lipscomb, CPA*ABV, ASA

Decosimo Corporate Finance

900 Tallan Building
2 Union Square
Chattanooga, TN 37402
Fax 423.266.6671

   Are there any known bugs revolving around the EU timezone not reverting from BST back to GMT?

[root at server2 ~]# rm -f /etc/localtime ; ln -s /usr/share/zoneinfo/Europe/London /etc/localtime
[root at server2 ~]# date
Wed Aug 29 21:42:31 BST 2007
[root at server2 ~]# date -s "Sun Oct 26 00:59:59 2008"
Sun Oct 26 00:59:59 BST 2008
[root at server2 ~]# date
Sun Oct 26 00:59:59 BST 2008
[root at server2 ~]# date
Sun Oct 26 01:00:01 BST 2008
[root at server2 ~]# date
Sun Oct 26 01:00:02 BST 2008
[root at server2 ~]# date
Sun Oct 26 01:00:03 BST 2008
[root at server2 ~]# date
Sun Oct 26 01:00:04 BST 2008
[root at server2 ~]# date -s "Sun Oct 26 01:00:01 2008"
Sun Oct 26 01:00:01 GMT 2008

Dafydd Rhys-Jones  |  Software Test Engineer
F5 Networks
  P 206.272.5555
 F 206.272.5556
  D 206.272.6280
M 206.335.1096


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