RE: Unexpected behaviour in date; calculates daylight savings tim e in correctly...

[Karl Zaryski]

Howdy...  Since I'd already worked up a workaround, I took your word for it
below.  Today, though, I'm working on a log-processing script that deals
with times, and I've been running into related trouble.  Trying the command
line you suggested...

        date ; date -d '01/01/1970 utc '`date -u +%s`' secs'
        Sun Jun  3 12:45:20 EDT 2001
        Sun Jun  3 13:45:20 EDT 2001

So, putting in the -u flag didn't help.  I tried testing several systems
with various architectures and operating systems, and -u didn't make a
difference anywhere.  The only system that gave the correct date both times
was a recent Sun box; an older Sun box and all PC boxes gave times one hour
off, regardless of use of the -u flag.

While searching on the net, I found evidence that other people have noticed
this problem... see
http://www.moongroup.com/archives/shell.scripting/2000-06/msg00047.html

Here are the results from trying this on various systems:

-=-

bash-2.03# uname -a
SunOS brother 5.8 Generic_108528-06 sun4u sparc SUNW,UltraSPARC-IIi-cEngine
bash-2.03# pwd
/tmp/sh-utils-2.0
bash-2.03# src/date --version
date (GNU sh-utils) 2.0
Written by David MacKenzie.

Copyright (C) 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
bash-2.03# src/date ; src/date -d '01/01/1970 utc '`src/date -u +%s`' secs'
Sun Jun  3 13:12:31 EDT 2001
Sun Jun  3 13:12:31 EDT 2001
bash-2.03# src/date ; src/date -d '01/01/1970 utc '`src/date +%s`' secs'
Sun Jun  3 13:12:36 EDT 2001
Sun Jun  3 13:12:36 EDT 2001

-=-

bash-2.03# uname -a
FreeBSD big.blackberry.net 4.0-RELEASE FreeBSD 4.0-RELEASE #0: Fri Jun 23
14:10:31 EDT 2000     address@hidden:/usr/src/sys/compile/LOCAL
i386
bash-2.03# pwd
/tmp/sh-utils-2.0
bash-2.03# src/date --version
date (GNU sh-utils) 2.0
Written by David MacKenzie.

Copyright (C) 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
bash-2.03# src/date ; src/date -d '01/01/1970 utc '`src/date -u +%s`' secs'
Sun Jun  3 13:10:20 EDT 2001
Sun Jun  3 14:10:21 EDT 2001
bash-2.03# src/date ; src/date -d '01/01/1970 utc '`src/date +%s`' secs'
Sun Jun  3 13:10:46 EDT 2001
Sun Jun  3 14:10:46 EDT 2001

-=-

address@hidden bb]$ uname -a
Linux grp01ykf.blackberry.net 2.4.5 #2 SMP Sun Jun 3 03:22:05 EDT 2001 i686
unknown
address@hidden bb]$ date --version
date (GNU sh-utils) 2.0
Written by David MacKenzie.

Copyright (C) 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
address@hidden bb]$ date ; date -d '01/01/1970 utc '`date -u +%s`' secs'
Sun Jun  3 13:16:28 EDT 2001
Sun Jun  3 14:16:28 EDT 2001
address@hidden bb]$ date ; date -d '01/01/1970 utc '`date +%s`' secs'
Sun Jun  3 13:16:33 EDT 2001
Sun Jun  3 14:16:33 EDT 2001

-=-

leela [371]$ uname -a
SunOS leela 5.5.1 Generic_103641-31 i86pc i386 i86pc
leela [372]$ pwd
/tmp/knz/sh-utils-2.0
leela [373]$ src/date --version
date (GNU sh-utils) 2.0
Written by David MacKenzie.

Copyright (C) 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
leela [374]$ src/date ; src/date -d '01/01/1970 utc '`src/date -u +%s`'
secs'
Sun Jun  3 13:23:48 EDT 2001
Sun Jun  3 14:23:48 EDT 2001
leela [375]$ src/date ; src/date -d '01/01/1970 utc '`src/date +%s`' secs'
Sun Jun  3 13:23:53 EDT 2001
Sun Jun  3 14:23:53 EDT 2001

-=-

@calum[375]$ uname -a
SunOS calum 5.7 Generic_106541-10 sun4m sparc SUNW,SPARCstation-10
@calum[376]$ pwd
/tmp/knz/sh-utils-2.0
@calum[377]$ src/date --version
date (GNU sh-utils) 2.0
Written by David MacKenzie.

Copyright (C) 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
@calum[378]$ src/date ; src/date -d '01/01/1970 utc '`src/date -u +%s`'
secs'
Sun Jun  3 13:55:51 EDT 2001
Sun Jun  3 14:55:51 EDT 2001
@calum[379]$ src/date ; src/date -d '01/01/1970 utc '`src/date +%s`' secs'
Sun Jun  3 13:55:57 EDT 2001
Sun Jun  3 14:55:57 EDT 2001

-----Original Message-----
From: Jim Meyering [mailto:address@hidden
Sent: Saturday, May 19, 2001 2:39 AM
To: Karl Zaryski
Cc: 'address@hidden'
Subject: Re: Unexpected behaviour in date; calculates daylight savings
time in correctly...


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
          120

     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
          18120

   * 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
          946684800

     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