GNU bug report logs - #38003
date --date=-1month gives same month today

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

From: Ilja Honkonen <ilja.honkonen <at> fmi.fi>
To: bug-coreutils <at> gnu.org
Subject: date --date=-1month gives same month today
Date: Thu, 31 Oct 2019 10:34:45 +0200

Hello
Please CC me as I'm not on this list. Running date (GNU coreutils) 8.26 
on fedora 30 today (date --utc  -I: 2019-10-31) with --date=-1month gives 
the same month which doesn't make sense:
$ date --utc -I --date=-1month
2019-10-01
I assume using --date=-31day also wouldn't work if run e.g. in beginning 
of march so this looks like a bug. Same happens also without --utc and 
-I. Thanks
Ilja

Message #8 received at 38003 <at> debbugs.gnu.org (full text, mbox):

From: Steven Hilton <malvidin <at> gmail.com>
To: Ilja Honkonen <ilja.honkonen <at> fmi.fi>
Cc: 38003 <at> debbugs.gnu.org
Subject: Re: bug#38003: date --date=-1month gives same month today
Date: Thu, 31 Oct 2019 17:25:53 +0100

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

There will be a discontinuity as long as the months are different lengths.

In the current implementation there will be only one discontinuity per
month; the 1st of the month always goes back to the 1st of the preceding
month. The overlap into the next month continues linearly until "now" is
the 1st of the month.

If I understand correctly, you would prefer that on March 29th, 30th, and
31st, this command would return the same date (except on leap years)?
$ date --date=-1month +"%m-%d"
02-28
This would introduce one to three discontinuities per month, but the
calculated month difference always the preceding month.


On Thu, Oct 31, 2019 at 3:46 PM Ilja Honkonen <ilja.honkonen <at> fmi.fi> wrote:

> Hello
> Please CC me as I'm not on this list. Running date (GNU coreutils) 8.26
> on fedora 30 today (date --utc  -I: 2019-10-31) with --date=-1month gives
> the same month which doesn't make sense:
> $ date --utc -I --date=-1month
> 2019-10-01
> I assume using --date=-31day also wouldn't work if run e.g. in beginning
> of march so this looks like a bug. Same happens also without --utc and
> -I. Thanks
> Ilja
>
>
>
>

[Message part 2 (text/html, inline)]

Message #11 received at 38003 <at> debbugs.gnu.org (full text, mbox):

From: Assaf Gordon <assafgordon <at> gmail.com>
To: Ilja Honkonen <ilja.honkonen <at> fmi.fi>, 38003 <at> debbugs.gnu.org
Subject: Re: bug#38003: date --date=-1month gives same month today
Date: Thu, 31 Oct 2019 11:35:46 -0600

tag 38003 notabug
close 38003
stop

Hello,

On 2019-10-31 2:34 a.m., Ilja Honkonen wrote:
> Please CC me as I'm not on this list. Running date (GNU coreutils) 8.26 
> on fedora 30 today (date --utc  -I: 2019-10-31) with --date=-1month 
> gives the same month which doesn't make sense:
> $ date --utc -I --date=-1month
> 2019-10-01

date gained a "--debug" option that helps diagnosing the issue:

$ date --utc -I --debug --date=-1month
date: parsed relative part: -1 month(s)
[...]
date: using current date as starting value: '(Y-M-D) 2019-10-31'
[...]
date: warning: when adding relative months/years, it is recommended to 
specify the 15th of the months           <----
date: after date adjustment (+0 years, -1 months, +0 days),
date:     new date/time = '(Y-M-D) 2019-10-01 17:29:20'
date: warning: month/year adjustment resulted in shifted dates:
date:      adjusted Y M D: 2019 09 31    <----
date:    normalized Y M D: 2019 10 01    <----
[...]
date: final: (Y-M-D) 2019-10-01 17:29:20 (UTC)
2019-10-01

------

Subtracting 1 month from October 31st results in September 31st.
Since the date doesn't exist, it is normalized:
September 31st is "one day after September 30th", which
results in October 1st.

The "--debug" option also warns: when subtracting months,
it is recommended to specify the 15th (middle) of the month,
exactly to avoid such issues.

   $ date --utc -I --date="2019-10-15 -1month"
   2019-09-15

regards,
 - assaf

Message #18 received at 38003 <at> debbugs.gnu.org (full text, mbox):

From: Ilja Honkonen <ilja.honkonen <at> fmi.fi>
To: Assaf Gordon <assafgordon <at> gmail.com>, 38003 <at> debbugs.gnu.org
Subject: Re: bug#38003: date --date=-1month gives same month today
Date: Wed, 6 Nov 2019 16:05:17 +0200

> Subtracting 1 month from October 31st results in September 31st.
> Since the date doesn't exist, it is normalized:
> September 31st is "one day after September 30th", which
> results in October 1st.

Thanks for explanation. To me it makes more sense that going one month 
back from 31 oct, or last day of oct, would give last day of sep instead 
of first day of oct. But now that I'm aware of this feature I can work 
around it if needed.
Ilja

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