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Re: Unexpected behaviour in date; calculates daylight savings time in co

From: Jim Meyering
Subject: Re: Unexpected behaviour in date; calculates daylight savings time in correctly...
Date: 19 May 2001 08:39:10 +0200
User-agent: Gnus/5.090003 (Oort Gnus v0.03) Emacs/21.0.104

Karl Zaryski <address@hidden> wrote:
| Hello!  I may have found a bug in the GNU date.  I expected that the
| following statements would give the same result:
|               date ; date -d '01/01/1970 utc '`date +%s`' secs'
| But the results are off by an hour:
|               Wed May 16 12:38:31 EDT 2001
|               Wed May 16 13:38:31 EDT 2001
| I haven't delved into the source code for date (I'm no expert programmer)
| but I suspect that the problem may be because I'm starting with a date out
| of daylight savings time (start of epoch) and adding a number of seconds to
| arrive at a result within daylight savings time... somewhere it adds the
| extra hour.  I've worked out a workaround for my scripts, but it's a bit of
| a kludge.
| Is this a bug?  If not, can you explain this unexpected behaviour?

It's not a bug.
But it is a little tricky.  Your inner date command needs -u.

  $ date ; date -d '01/01/1970 utc '`date -u +%s`' secs'
  Sat May 19 08:34:39 CEST 2001
  Sat May 19 08:34:39 CEST 2001

BTW, there are some examples in the documentation that show how to do that.

[the following is at the bottom of the section you see if you run
 the command `info date example': ]

   * To convert a date string to the number of seconds since the epoch
     (which is 1970-01-01 00:00:00 UTC), use the `--date' option with
     the `%s' format.  That can be useful in sorting and/or graphing
     and/or comparing data by date.  The following command outputs the
     number of the seconds since the epoch for the time two minutes
     after the epoch:

          date --date='1970-01-01 00:02:00 +0000' +%s

     If you do not specify time zone information in the date string,
     `date' uses your computer's idea of the time zone when
     interpreting the string.  For example, if your computer's time
     zone is that of Cambridge, Massachusetts, which was then 5 hours
     (i.e., 18,000 seconds) behind UTC:

          # local time zone used
          date --date='1970-01-01 00:02:00' +%s

   * If you're sorting or graphing dated data, your raw date values may
     be represented as seconds since the epoch.  But few people can
     look at the date `946684800' and casually note "Oh, that's the
     first second of the year 2000 in Greenwich, England."

          date --date='2000-01-01 UTC' +%s

     To convert such an unwieldy number of seconds back to a more
     readable form, use a command like this:

          # local time zone used
          date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
          1999-12-31 19:00:00 -0500

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