bug#11115: linux date arithmetic

[Ruediger Meier]

On Wednesday 28 March 2012, Eric Blake wrote:
> [adding bug-gnulib]
>
> On 03/28/2012 06:39 AM, Stefan Karamuz wrote:
> > Please check the 2 linux commands:
> >
> > date -d "$(date +%F\ %H:%M:%S)" +%F\ %H:%M:%S
> > date -d "$(date +%F\ %H:%M:%S) + 1 minute" +%F\ %H:%M:%S
> >
> > It's very confusing, because the results of the two commands differ
> > in one hour and one minute, except of one minute only.
> >
> > [~]$ date -d "$(date +%F\ %H:%M)" +%F\ %H:%M:%S
> > 2012-03-28 14:06:00
> >
> > ... and after a few seconds:
> >
> > [~]$ date -d "$(date +%F\ %H:%M:%S) + 1 minute" +%F\ %H:%M:%S
> > 2012-03-28 15:07:20
>
> Thanks for the report.  It's easier to debug if you use the same
> starting point for both commands rather than nesting an ever-changing
> $(date) to feed the outermost 'date -d', but I can indeed reproduce
> your behavior, as well as explain it:
>
> $ date -d '2012-03-28 11:38'            +%H:%M:%S
> 11:38:00
>
> Starting point.
>
> $ date -d '2012-03-28 11:38 +1'         +%H:%M:%S
> 04:38:00
>
> Aha - '11:38 +1' is parsed as a time zone designation.  In my case,
> since I'm in UTC-6, or 7 time zones away from UTC+1, that explains
> why my output jumped backwards by 7 hours; I'm assuming your location
> is at UTC+2.
>
> $ date -d '2012-03-28 11:38 +minute'    +%H:%M:%S
> 11:39:00
>
> Notice that 'minute' can be parsed without units.

BTW I think that the parser syntax for -d got very unlucky implemented. 
IMO dates and intervals should have been clearly separated somehow. For 
me the parser is too much intelligent and does not what the user would 
expect.

Maybe there should be a separate option --add which takes only intervals 
(negative or positive) to be added to the stamp comming from -r,-d,-f 
or now.

cu,
Rudi

> $ date -d '2012-03-28 11:38 + 1 minute' +%H:%M:%S
> 04:39:00
>
> Therefore, the parser is faced with an ambiguity between:
>
> (11:38 +1) minute
> 11:38 (+1 minute)
>
> and happened to choose the result you consider to be less intuitive.
> But looks what happen if you rearrange the command line:
>
> $ date -d '+1 minute 2012-03-28 11:38'  +%H:%M:%S
> 11:39:00
>
> Now there is no ambiguity.
>
> I don't know if this is something adequately explained in our FAQ:
> https://www.gnu.org/software/coreutils/faq/#The-date-command-is-not-w
>orking-right_002e
>
> nor do I know if it might be possible to patch gnulib's getdate.y to
> make the parsing smarter, but you are welcome to try patching it.  In
> fact, I'd go so far as to say getdate.y has a bug; '+1' is a
> reasonable time zone designator when it occurs after hh:mm, but I've
> never seen '+ 1' meant as a time zone, so it seems like the presence
> of space after '+' should be used to alter the parse precedence.
>
> So I'm not sure whether to leave this bug open (in hopes of someone
> writing a patch) or to close it (since the behavior is explainable,
> even if not what you wanted, and can be worked around with a little
> reordering on your part).