GNU bug report logs - #18159
Bugin getting 'last month' on last day of the month

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 18159 in the body.
You can then email your comments to 18159 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

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

From: jemail2013 <at> comcast.net
To: bug-coreutils <at> gnu.org
Subject: Bugin getting 'last month' on last day of the month
Date: Thu, 31 Jul 2014 16:10:22 +0000 (UTC)

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

Hi, 
Today is July 31st. I am trying to get the last month and the result is still 07 which is the current month. Also, the day for the last month is returned as '01'. 
This worked correctly yesterday, July 30th. So, I think that there is a bug running it on the last day of the month. 
Either that, I am doing something silly :-). 

But please check. 

[M1]->date +'%Y%m%d' 
20140731 


[M1]->date +'%Y%m%d' -d ' last month ' 
20140701 

[M1]->date +'%Y%m%d' -d ' 2 months ago ' 
20140531 




Thanks, 


Murali. 

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

Message #8 received at 18159 <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: jemail2013 <at> comcast.net
Cc: 18159 <at> debbugs.gnu.org
Subject: Re: bug#18159: Bugin getting 'last month' on last day of the month
Date: Fri, 01 Aug 2014 11:43:48 +0100

On 07/31/2014 05:10 PM, jemail2013 <at> comcast.net wrote:
> Hi, 
> Today is July 31st. I am trying to get the last month and the result is still 07 which is the current month. Also, the day for the last month is returned as '01'. 
> This worked correctly yesterday, July 30th. So, I think that there is a bug running it on the last day of the month. 
> Either that, I am doing something silly :-). 
> 
> But please check. 
> 
> [M1]->date +'%Y%m%d' 
> 20140731 
> 
> 
> [M1]->date +'%Y%m%d' -d ' last month ' 
> 20140701 
> 
> [M1]->date +'%Y%m%d' -d ' 2 months ago ' 
> 20140531 

This issue with relative date calculations is documented at:
http://www.gnu.org/software/coreutils/faq/coreutils-faq.html#The-date-command-is-not-working-right_002e

The issue is that last month isn't context sensitive and will
substract a fixed amount of time, not appropriate at the end of all months.
So simplifying your example a bit, you get this issue:

  $ date
  Thu Jul 31 11:35:29 IST 2014
  $ date +'%m'
  07
  $ date +'%m' -d ' last month '
  07

The same thing happens with `date -d yesterday +%F` going back 2 days around DST.

Personally I do think that's a bug that we could avoid.
I.E. the `date` command should be doing higher level operations with relative items.
I.E. it should not simply degenerate 1 day to 86400 seconds, etc.
Logic would be something like:

  if (relative_input)
    {
      relative_unit = HCF (relative_input_unit, output_unit);
      start_adjustment = get_middle_of_unit (relative_unit);
    }
  else
    start_adjustment = 0;
  if relative_unit > 0
    start_adjustment = - start_adjustment;
  start += start_adjustment;

I can't see anyone depending on the problematic old behavior?

thanks,
Pádraig.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.