[tz] [PROPOSED] date +%z now outputs -0000 for uninhabited

[Jon Skeet]

I think that might give cause for confusion with respect to RFC3339 style
string formatting, section 4.3:

4.3. Unknown Local Offset Convention

If the time in UTC is known, but the offset to local time is unknown,
> this can be represented with an offset of "-00:00". This differs
> semantically from an offset of "Z" or "+00:00", which imply that UTC
> is the preferred reference point for the specified time.

That's not what we intend to convey in this case, is it?

I realize that the we're not trying to output RFC3339, but I think it
could lead to confusion.

Jon


On 15 January 2017 at 08:48, Paul Eggert <eggert at cs.ucla.edu> wrote:

> When the time zone has UT offset 0 and the time zone abbreviation
> begins with "-", the %z spec now expands to -0000 instead of to
> +0000.  This conveys more-useful info, and is what the next
> version of GNU 'date' is intended to do.
> * NEWS, newstrftime.3: Document change.
> * date.1: Document %z.
> * strftime.c (_fmt): Implement change.
> ---
>  NEWS          |  4 ++++
>  date.1        |  1 +
>  newstrftime.3 | 12 ++++++++++++
>  strftime.c    | 13 ++++++++++++-
>  4 files changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/NEWS b/NEWS
> index 8276cc7..3975295 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -74,6 +74,10 @@ Unreleased, experimental changes
>      when TZ is set to a POSIX-style string that specifies DST.
>      (Problem reported by Kees Dekker.)
>
> +    date and strftime now cause %z to generate "-0000" instead of
> +    "+0000" when the UT offset is zero and the time zone abbreviation
> +    begins with "-".
> +
>    Changes to documentation and commentary
>
>      tz-link.htm now covers leap smearing, which is popular in clouds.
> diff --git a/date.1 b/date.1
> index 7fd4848..579ab92 100644
> --- a/date.1
> +++ b/date.1
> @@ -90,6 +90,7 @@ to be output in a particular way
>  %X     14:54:40        Time*
>  %y     89      Last two digits of year
>  %Y     1989    Year in full
> +%z     -0500   Numeric time zone
>  %Z     EST     Time zone abbreviation
>  %+     Wed Mar  8 14:54:40 EST 1989    Default output format*
>  .if t .in -.5i
> diff --git a/newstrftime.3 b/newstrftime.3
> index 9a60ee3..8f1be34 100644
> --- a/newstrftime.3
> +++ b/newstrftime.3
> @@ -50,6 +50,13 @@ strftime \- format date and time
>  .SH DESCRIPTION
>  .ie '\(en'' .ds en \-
>  .el .ds en \(en
> +.ie '\(lq'' .ds lq \&"\"
> +.el .ds lq \(lq\"
> +.ie '\(rq'' .ds rq \&"\"
> +.el .ds rq \(rq\"
> +.de q
> +\\$3\*(lq\\$1\*(rq\\$2
> +..
>  The
>  .I strftime
>  function formats the information from
> @@ -211,6 +218,11 @@ is replaced by the offset from the Prime Meridian
>  in the format +HHMM or \*-HHMM as appropriate,
>  with positive values representing locations east of Greenwich,
>  or by the empty string if this is not determinable.
> +The numeric time zone \*-0000 is used when the time is Universal Time
> +but local time is indeterminate; by convention this is used for
> +locations while uninhabited, and corresponds to a zero offset when the
> +time zone abbreviation begins with
> +.q "\*-" .
>  .TP
>  %%
>  is replaced by a single %.
> diff --git a/strftime.c b/strftime.c
> index 984ead5..2bfb6c1 100644
> --- a/strftime.c
> +++ b/strftime.c
> @@ -500,6 +500,7 @@ label:
>                                 {
>                                 long            diff;
>                                 char const *    sign;
> +                               bool negative;
>
>  # ifdef TM_GMTOFF
>                                 diff = t->TM_GMTOFF;
> @@ -538,7 +539,17 @@ label:
>                                         continue;
>  #  endif
>  # endif
> -                               if (diff < 0) {
> +                               negative = diff < 0;
> +                               if (diff == 0) {
> +#ifdef TM_ZONE
> +                                 negative = t->TM_ZONE[0] == '-';
> +#else
> +                                 negative
> +                                   = (t->tm_isdst < 0
> +                                      || tzname[t->tm_isdst != 0][0] ==
> '-');
> +#endif
> +                               }
> +                               if (negative) {
>                                         sign = "-";
>                                         diff = -diff;
>                                 } else  sign = "+";
> --
> 2.7.4
>
>
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