Re: Parsing Date Strings with Time Zones in c++

James Kanze <>
Sat, 9 Oct 2010 04:11:05 -0700 (PDT)
On Oct 9, 8:32 am, Jorgen Grahn <> wrote:

On Fri, 2010-10-08, James Kanze wrote:

On Oct 8, 1:48 pm, Jorgen Grahn <> wrote:

On Wed, 2010-10-06, Goran wrote:

On Oct 6, 8:28 am, codejockey <> wrote:



I *think* that on Linux you can at least say "I have this
time in my current time zone; please give me UTC time" and
get the right answer (except for times that happen during
the DST-duplicated hour in fall and the lost hour in
spring) by passing -1 as the TZ in some system call (I
forget which, am too lazy to look it up, and too lazy to
check if POSIX makes guarantees here).

I was thinking of mktime(3), and passing -1 in struct tm::tm_isdst.

As has been pointed out, it's simply not possible, even if the
manual claims to do it. To determine the right answer, you have
to know whether summer time is in effect or not, and to know
that, you have to know the jurisdiction, in addition to the time

I admit I read the thread sloppily ... Are you essentially
saying you cannot convert the brief timezone info in a textual
timestamp to system timezone info, because the former may
cover many jurisdictions?


(If you *do* know the jurisdiction you should be OK. Timezone info (on
my machine) *does* have knowledge of plenty of jurisdictions;
/usr/share/zoneinfo is full of (among other things) DST rules for
places like restricted parts of Tasmania, a hut on the Bailey
Peninsula of Antarctica and so on.)

If you know the jurisdiction (and have all of the data files
installed), you're OK, but unlike the time zone, the
jurisdiction typically isn't part of the time and date string.

In the US, I believe that it is common to use things like EDT
instead of EST when you're in summer time. This is probably the
best approach (the sender knows whether he is in summer time or
not), but it's far from universal---most of the time, in Europe,
I've seen CET (central European time) used, regardless of
whether we're in summer time or not. (The Wikipedia gives CEST
and CEDT, but I've never seen either in actual use. I have
seen, occasionally, software which claims EET for France when
we're in summer time, but this seems to be the exception rather
than the rule. And leads to further confusion: is this EET
somewhere in eastern Europe, with summer time in effect, so
+0300, or is central Europe faking it, and +0200.)

James Kanze

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