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winsup/doc/faq-api.xml(faq.api.timezone): explain time zone updates 

based on material from tz@IANA.org mailing list sources
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Brian Inglis 2020-08-27 01:17:09 -06:00 committed by Corinna Vinschen
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winsup/doc/faq-api.xml
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@ -385,13 +385,43 @@ Cygwin version number details, check out the
</answer></qandaentry>
<qandaentry id="faq.api.timezone">
<question><para>Why isn't timezone set correctly?</para></question>
<question><para>Why isn't my time (or zone) set correctly?</para></question>
<answer>
<para><emphasis role='bold'>(Please note: This section has not yet been updated for the latest net release.)</emphasis>
<para>Daylight saving (Summer time) and other time zone changes are
decided on by politicians, and announced by government officials,
sometimes with short or no notice, so time zone updates are released at
least a few, and sometimes several, times a year.
Details of changes are not known until they are announced publicly by
officials, often in foreign languages.
Those details then have to be noticed, possibly translated, passed to,
picked up, and applied by the official <filename>tzdata</filename>
source package maintainers.
That information has to be compiled, checked, and released publicly in
an update to the official <filename>tzdata</filename> source package.
Then those changes have to be picked up and applied to the Cygwin
<filename>tzdata</filename> package, which has to be updated, built,
tested, and released publicly.
</para>
<para>Did you explicitly call tzset() before checking the value of timezone?
If not, you must do so.
<para>Time zone settings are updates to the daylight saving (Summer
time) rules for dates of changes, hour offsets from UTC of time zones,
and the geographic regions to which those rules and offsets apply,
provided in the <filename>tzdata</filename> package included in all
Cygwin installations.
Have you run the Cygwin Setup program recently to update at least
the <filename>tzdata</filename> package?
</para>
<para>Are you developing applications using times which may be affected
by time zones?
Since the <literal>ctime()</literal>, <literal>localtime()</literal>,
<literal>mktime()</literal>, and <literal>strftime()</literal> functions
are required to set time zone information as if by calling
<literal>tzset()</literal>, there is no need for an explicit
<literal>tzset()</literal> call before using these functions.
However, if none of the above functions are called first, applications
should ensure <literal>tzset()</literal> is called explicitly before
using any other time functions, or checking or using time zone
information.
</para>
</answer></qandaentry>