Initialize time zone information before using it in wxGetTimeZone().

We must call _tzset() before calling _get_timezone() as while this is normally
done implicitly by the other time functions, it might not have been done yet
if create a wxDateTime::TimeZone before calling any of them.

Closes #12700.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66241 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/datetime.cpp

@@ -142,6 +142,10 @@ wxCUSTOM_TYPE_INFO(wxDateTime, wxToStringConverter<wxDateTime> , wxFromStringCon
         // deprecated and _get_timezone() should be used instead.
         static long wxGetTimeZone()
         {
+            // We must initialize the time zone information before using it
+            // (this will be done only once internally).
+            _tzset();
+
             long t;
             _get_timezone(&t);
             return t;