bug#13058: European date format

[Bob Proulx]

Ohad Basan wrote:
> Hey

Hey back!

> Regarding 'date' command.
> the -d parameter doesn't know how to receive a european date format
> e.g. DD/MM/YYYY

I am assuming by "doesn't know how to receive" you mean it produces a
different result than one you wish?  Because of course it parses it.
It just parses it (receives it) with a different result than you
wish.  Please give an example of the behavior you see and what you
would like to see instead.  Did you mean this:

  $ date -d '12/15/2012 12:00' '+%F %T %z'
  2012-12-15 12:00:00 -0700

Or this:

  $ date -d '15/12/2012 12:00' '+%F %T %z'
  date: invalid date ‘15/12/2012 12:00’

> I believe that it is a bug.

A typical format for digit/digit/digit is the US MM/DD/YYYY format.
Parsing the date as that format works fine.

  $ date -R -d '12/02/2012 12:00'
  Sun, 02 Dec 2012 12:00:00 -0700

  $ date -R -u -d '12/02/2012'
  Sun, 02 Dec 2012 00:00:00 +0000

Also using the truly ambiguous 2-digit year.  Example of bad example.

  $ date -R -d '12/02/12 12:00'
  Sun, 02 Dec 2012 12:00:00 -0700

  $ date -R -u -d '12/02/12'
  Sun, 02 Dec 2012 00:00:00 +0000

How would you distinguish between MM/DD/YYYY and DD/MM/YYYY?

I think it is much, much better to avoid the ambiguity entirely by
using one of the standard date formats rather than a locale specific
format.  As you can see in the above I always used the -R, --rfc-2822
option to produce standard and unambiguous formats.

Alternatives

I am very much an advocate of the '+%F %T %z' format which also
produces a stable result.

  $ date '+%F %T %z'
  2012-12-02 14:21:44 -0700

  $ date -R -d '2012-12-02 14:21:44 -0700'
  Sun, 02 Dec 2012 14:21:44 -0700

Note: Always either specify the time of day such as 12:00 as I did
above or specify use of UTC such as I did above with -u to avoid
problems with daylight savings time.

I also mention --rfc-3339 which is very useful in producing standard
unambiguous time strings.

      --rfc-3339=TIMESPEC   output date and time in RFC 3339 format.
                            TIMESPEC='date', 'seconds', or 'ns' for
                            date and time to the indicated precision.
                            Date and time components are separated by
                            a single space: 2006-08-07 12:34:56-06:00

  $ date --rfc-3339=date
  2012-12-02

  $ date --rfc-3339=seconds
  2012-12-02 14:34:38-07:00

  $ date --rfc-3339=ns
  2012-12-02 14:34:40.670385314-07:00

All of which may be parsed with date -d.

  $ date -d "2012-12-02 14:34:40.670385314-07:00" '+%F %T.%N %z'
  2012-12-02 14:34:40.670385314 -0700

Please also see the FAQ for date as there is much discussion there of
these and other issues.

  
http://www.gnu.org/software/coreutils/faq/coreutils-faq.html#The-date-command-is-not-working-right_002e

Bob