Javascript date function

Dave Higton dave at davehigton.me.uk
Sun Sep 20 21:52:05 BST 2015


In message <5eb0fefc54.DaveMeUK at my.inbox.com>
          Dave Higton <dave at davehigton.me.uk> wrote:

>I don't know whether this is relevant at this stage.
>
>I've been putting together a few noddy Javascript examples.  I noticed
>tonight that the date function shows the time zone as being +02:00 and
>shows the time as being 2 hours later than UTC.  I'm wondering if it
>is adding DST to a time that already has DST.  My Iyonix is completely
>set up for the UK.  AFAIK it's the settings you'd expect anyone in the
>UK to have.
>
>NS #2953.

Right, I've tracked this down... sort of.

In duktape.c, function duk_bi_date_get_local_tzoffset_gmtime() is
called, but returns 7200.

The salient pints are:

gmtime_r(&t, &tms[0]);
localtime_r(&t, &tms[1]);

/* At ths point, tms[0].tm_hour is the UTC hour, tms[0].tm_isdst is 0;
  tms[1].tm_hour is the local hour (1 greater than the UTC hout 'cos
  we're currently in daylight savings), and tms[1].tm_isdst = 1.
  I think those values are what one would expect. */

t1 = mktime(&tms[0]);
t2 = mktime(&tms[1]);

if (tms[1].tm_isdst > 0) {
 t2 += 3600;
}

/* Now why on earth do they do that?  t2 is a local time, and as such
is already corrected for DST - shouldn't it be? */

return (t2 - t1);

This is around lines 27864 to 27910 in duktape.c.

I'm not finding clear answers on the Internet as to whether localtime_r()
should return numbers that already directly represent the local legal
time, or the non-DST time in that time zone and expect the tm_isdst flag
to be used to add an hour offset.

Dave

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