On Tue 06 Feb 2018 at 13:48:19 (+0100), Vincent Lefevre wrote:
> On 2018-02-05 09:39:12 -0500, Greg Wooledge wrote:
> > (*) One specific shell script use case was "Get the last date of a given
> > month." Now, obviously you can just set up an array of hard-coded month
> > ending dates, and then write a function to determine whether the current
> > year is a leap year for the February case. But if you want to do it with
> > GNU date -d, then you have to guess a magic incantation that actually
> > works. Usually by trial and error.
> >
> > Anyway, here's what I came up with:
> >
> > lastday() {
> > date +%Y-%m-%d -d "$1 1 day ago + 1 month"
> > }
>
> But the exact meaning of "month" seems undocumented, which may
> silently break in a future version (e.g. possibly as a consequence
> of a bug fix). So this is a good example of what you should *not*
> do.
>
> This is completely crazy:
>
> zira% date +%Y-%m-%d -d '2003-09-01 1 day ago + 1 month'
> 2003-09-30
Don't say you weren't warned; I demonstrated the --debug flag
(in https://lists.debian.org/debian-user/2018/02/msg00101.html)
which gives, for your example:
$ date +%Y-%m-%d -d '2003-09-01 1 day ago + 1 month' --debug
date: parsed date part: (Y-M-D) 2003-09-01
date: parsed relative part: -1 day(s)
date: parsed relative part: +1 month(s) -1 day(s)
date: input timezone: -06:00 (set from TZ="US/Central" environment value)
date: warning: using midnight as starting time: 00:00:00
date: starting date/time: '(Y-M-D) 2003-09-01 00:00:00 TZ=-06:00'
date: warning: when adding relative months/years, it is recommended to specify
the 15th of the months
date: warning: when adding relative days, it is recommended to specify 12:00pm
date: after date adjustment (+0 years, +1 months, -1 days),
date: new date/time = '(Y-M-D) 2003-09-30 00:00:00 TZ=-06:00'
date: '(Y-M-D) 2003-09-30 00:00:00 TZ=-06:00' = 1064898000 epoch-seconds
date: output timezone: -06:00 (set from TZ="US/Central" environment value)
date: final: 1064898000.000000000 (epoch-seconds)
date: final: (Y-M-D) 2003-09-30 05:00:00 (UTC0)
date: final: (Y-M-D) 2003-09-30 00:00:00 (output timezone TZ=-06:00)
2003-09-30
$
Most of the examples were begging for off-by-one errors.
Cheers,
David.
