Re: date-command

[Bob Proulx]

address@hidden wrote:
> I´m running coreutils 6.9.92.4-f088d-dirty (dirty??),

"dirty" means that you are running from a git version control system
checkout of the code with uncommitted changes and not from an official
upstream distribution image.  That is okay.

> and I found a bug. I´m using the function of asking for a date some
> month ago. For example date --date="-2 month" +%Y%m%d. And when
> doing that on the first o june 01:00, I got the answer march 31,
> instead of April 1.

This is a known limitation.  The date processing code was incorporated
from a best-effort module that isn't 100% precise.  Months are fuzzy
quantities that vary in duration.  The operation of the plus and minus
month adjustments is to add or subtract a month's worth of days.
However this may land the result on an invalid date or on a date that
isn't into the previous month.

See the Coreutils documentation section "Relative items in date
strings" where this is described in more detail.  Expecially this
section:

     The fuzz in units can cause problems with relative items.  For
  example, `2003-07-31 -1 month' might evaluate to 2003-07-01, because
  2003-06-31 is an invalid date.  To determine the previous month more
  reliably, you can ask for the month before the 15th of the current
  month.  For example:

       $ date -R
       Thu, 31 Jul 2003 13:02:39 -0700
       $ date --date='-1 month' +'Last month was %B?'
       Last month was July?
       $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
       Last month was June!

> It doesn´t count a number of month backwords. But a number of days,
> not calculating correctly how many days the different days have.

That is correct.  The recommended proceedure is to do month
calculations based upon the middle of the month.  Normally I first get
the time once with date and then use it as a reference time thereafter
so as to avoid issues with crossing midnight on a month boundary.

>  This is my first bug-report, so I hope I reported in a good manner.

Your report was great!

Bob