How to convert time from one timezone to another?

Hi,

I need help in time conversion from one time zone to another
programmatically.
For e.g With Local Time zone as (GMT -5:00) US Eastern time I want to
convert time 29 Mar 2008 00:00:00 on TimeZone(GMT+1:00) into UTC.

I tried first converting the time into time_t using mktime, but
time_t uses local timezone and I could not find any API wherein I can
specify time as well as timezone.

Thanks in advance,
Art

Igor
Thu Jul 24 10:04:08 CDT 2008

Art <artami2006@gmail.com> wrote:
> I need help in time conversion from one time zone to another
> programmatically.
> For e.g With Local Time zone as (GMT -5:00) US Eastern time I want to
> convert time 29 Mar 2008 00:00:00 on TimeZone(GMT+1:00) into UTC.

If you know it's GMT+1:00, can't you just subtract one hour? Then call
mktime - it will normalize out-of-range values. Ignore the time_t return
value, you are calling mktime only for the side effect.
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925

Art
Thu Jul 24 10:17:05 CDT 2008

During DST transitions, I guess just substracting one hour may not
work. Is there any way to get the info from OS.

On Jul 24, 8:04=A0pm, "Igor Tandetnik" <itandet...@mvps.org> wrote:
> Art <artami2...@gmail.com> wrote:
> > I need help in time conversion from one time zone to another
> > programmatically.
> > For e.g With Local Time zone as (GMT -5:00) US Eastern time I want to
> > convert time 29 Mar 2008 00:00:00 on TimeZone(GMT+1:00) into UTC.
>
> If you know it's GMT+1:00, can't you just subtract one hour? Then call
> mktime - it will normalize out-of-range values. Ignore the time_t return
> value, you are calling mktime only for the side effect.
> --
> With best wishes,
> =A0 =A0 Igor Tandetnik
>
> With sufficient thrust, pigs fly just fine. However, this is not
> necessarily a good idea. It is hard to be sure where they are going to
> land, and it could be dangerous sitting under them as they fly
> overhead. -- RFC 1925

Doug
Thu Jul 24 10:19:32 CDT 2008

On Thu, 24 Jul 2008 07:51:09 -0700 (PDT), Art <artami2006@gmail.com> wrote:

>Hi,
>
>I need help in time conversion from one time zone to another
>programmatically.
>For e.g With Local Time zone as (GMT -5:00) US Eastern time I want to
>convert time 29 Mar 2008 00:00:00 on TimeZone(GMT+1:00) into UTC.
>
>I tried first converting the time into time_t using mktime, but
>time_t uses local timezone and I could not find any API wherein I can
>specify time as well as timezone.

You might be able to use _tzset for this. Also, you might consider using
the Windows FILETIME/SYSTEMTIME instead of time_t/struct tm. The Windows
API provides functions like TzSpecificLocalTimeToSystemTime, which allow
you to specify the time zone for the local time.

--
Doug Harrison
Visual C++ MVP

Igor
Thu Jul 24 10:29:05 CDT 2008

Art <artami2006@gmail.com> wrote:
> During DST transitions, I guess just substracting one hour may not
> work.

GMT+1:00 is, by definition, one hour ahead of UTC regardless of DST. It
is possible that some jurisdiction is in GMT+1:00 in winter and in
GMT+2:00 in summer. Is that what you mean? Then you need to specify
timezone in some geographical way, not by a GMT offset.

> Is there any way to get the info from OS.

You mean, the information on the DST rules of a particular jurisdiction?
Not with a public API, as far as I can tell. This information is stored
in the registry under

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time
Zones

in an undocumented format (which, reportedly, has changed between XP and
Vista).
--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925