GNU bug report logs - #15752
Date problem

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Andy Lamb <andy <at> andy-lamb.com>
To: bug-coreutils <at> gnu.org
Subject: Date problem
Date: Tue, 29 Oct 2013 21:58:05 -0400

Hello,

February of 2013 does not work, it comes up with March twice!

/bin/date --date=-6 month +%b%Y = Apr2013
/bin/date --date=-7 month +%b%Y = Mar2013
/bin/date --date=-8 month +%b%Y = Mar2013
/bin/date --date=-9 month +%b%Y = Jan2013

Neither of these work:

file /bin/date
/bin/date: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), 
dynamically linked (uses shared libs), for GNU/Linux 2.6.9, stripped

file /bin/date
/bin/date: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), 
dynamically linked (uses shared libs), for GNU/Linux 2.6.18, stripped

Thanks,
Andy

Message #12 received at 15752-done <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: Andy Lamb <andy <at> andy-lamb.com>, 15752-done <at> debbugs.gnu.org
Subject: Re: bug#15752: Date problem
Date: Tue, 29 Oct 2013 21:02:05 -0600

[Message part 1 (text/plain, inline)]

tag 15752 notabug
thanks

On 10/29/2013 07:58 PM, Andy Lamb wrote:
> Hello,
> 
> February of 2013 does not work, it comes up with March twice!
> 
> /bin/date --date=-6 month +%b%Y = Apr2013
> /bin/date --date=-7 month +%b%Y = Mar2013
> /bin/date --date=-8 month +%b%Y = Mar2013
> /bin/date --date=-9 month +%b%Y = Jan2013

This is a FAQ:

https://www.gnu.org/software/coreutils/faq/coreutils-faq.html#The-date-command-is-not-working-right_002e

Subtracting by months is fuzzy (does it mean multiples of 28, 29, 30, or
31 days?  What is one month after Oct 31, Nov 30 or Dec 1?).  Daylight
Savings doesn't help (even if I subtract an exact multiple of days,
there are cases where a timezone could cause a multiple of 24 hours to
bypass a 23-hour day or get trapped by a 25-hour day).  You are best off
trying to start from a middle of a month to guarantee that even with the
fuzz you still land in the desired destination.

$ date --date="$(date "+%b 15") -6 month" +%b%Y
Apr2013
$ date --date="$(date "+%b 15") -7 month" +%b%Y
Mar2013
$ date --date="$(date "+%b 15") -8 month" +%b%Y
Feb2013
$ date --date="$(date "+%b 15") -9 month" +%b%Y
Jan2013

As such, I'm closing this as not a bug, although you may feel free to
add further comments.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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