Re: bug#13071: Date and month ago

[Tom Roche]

http://lists.gnu.org/archive/html/bug-coreutils/2012-12/msg00003.html (Bob 
Proulx)
> it is often wise to adopt universal time by setting [`TZ=UTC0']
> before embarking on calendrical calculations.

> And also see the FAQ entry which also discusses this topic in some
> detail:
  
> http://www.gnu.org/software/coreutils/faq/#The-date-command-is-not-working-right_002e

> I am sure those docs answer your questions. But just the same we
> would appreciate a note from you saying that it did.

The above helped me to diagnose/fix the incorrect last day in Nov 2008
(below).

> Or suggesting improvements!  :-)

A --verbose switch on `date` that would do something like

MODEL_YEAR='2008'
for M in {1..12} ; do
  MM="$(printf '%02d' ${M})"
  FIRST_DAY_OF_MONTH="$(date --verbose -d "${MODEL_YEAR}-${MM}-01")"
  echo -e "first day of month=${FIRST_DAY_OF_MONTH}"
  LAST_DAY_OF_MONTH="$(date --verbose -d "${FIRST_DAY_OF_MONTH} + 1 month - 1 
day")"
  echo -e " last day of month=${LAST_DAY_OF_MONTH}"
  echo # newline
done
> first day of month=Tue Jan  1 00:00:00 EST 2008
>  last day of month=Thu Jan 31 00:00:00 EST 2008

> first day of month=Fri Feb  1 00:00:00 EST 2008
* date: <something about leap year/>
>  last day of month=Fri Feb 29 00:00:00 EST 2008

> first day of month=Sat Mar  1 00:00:00 EST 2008
* date: weirdness happened when calculating <whatever/>: <explanation/>
>  last day of month=Mon Mar 31 01:00:00 EDT 2008

> first day of month=Tue Apr  1 00:00:00 EDT 2008
>  last day of month=Wed Apr 30 00:00:00 EDT 2008

> first day of month=Thu May  1 00:00:00 EDT 2008
>  last day of month=Sat May 31 00:00:00 EDT 2008

> first day of month=Sun Jun  1 00:00:00 EDT 2008
>  last day of month=Mon Jun 30 00:00:00 EDT 2008

> first day of month=Tue Jul  1 00:00:00 EDT 2008
>  last day of month=Thu Jul 31 00:00:00 EDT 2008

> first day of month=Fri Aug  1 00:00:00 EDT 2008
>  last day of month=Sun Aug 31 00:00:00 EDT 2008

> first day of month=Mon Sep  1 00:00:00 EDT 2008
>  last day of month=Tue Sep 30 00:00:00 EDT 2008

> first day of month=Wed Oct  1 00:00:00 EDT 2008
>  last day of month=Fri Oct 31 00:00:00 EDT 2008

> first day of month=Sat Nov  1 00:00:00 EDT 2008
* date: weirdness happened when calculating <whatever/>: <explanation/>
>  last day of month=Sat Nov 29 23:00:00 EST 2008

> first day of month=Mon Dec  1 00:00:00 EST 2008
>  last day of month=Wed Dec 31 00:00:00 EST 2008

might help users debug. Note my fix (per your suggestions) was

MODEL_YEAR='2008'
for M in {1..12} ; do
  MM="$(printf '%02d' ${M})"
  FIRST_DAY_OF_MONTH="$(TZ='UTC0' date -d "${MODEL_YEAR}-${MM}-01")"
  echo -e "first day of month=${FIRST_DAY_OF_MONTH}"
  LAST_DAY_OF_MONTH="$(TZ='UTC0' date -d "${FIRST_DAY_OF_MONTH} + 1 month - 1 
day")"
  echo -e " last day of month=${LAST_DAY_OF_MONTH}"
  echo # newline
done

FWIW, Tom Roche <address@hidden>