Extracted private GetTimeZone() to public wxGetTimeZone() function.

Created new wx/time.h header for this function, more time-related functions
will be added to it later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69836 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/time.cpp

@@ -0,0 +1,100 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        src/common/time.cpp
+// Purpose:     Implementation of time-related functions.
+// Author:      Vadim Zeitlin
+// Created:     2011-11-26
+// RCS-ID:      $Id: wxhead.cpp,v 1.11 2010-04-22 12:44:51 zeitlin Exp $
+// Copyright:   (c) 2011 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+// ============================================================================
+// declarations
+// ============================================================================
+
+// ----------------------------------------------------------------------------
+// headers
+// ----------------------------------------------------------------------------
+
+// for compilers that support precompilation, includes "wx.h".
+#include "wx/wxprec.h"
+
+#ifdef __BORLANDC__
+    #pragma hdrstop
+#endif
+
+#include "wx/time.h"
+
+#ifndef WX_GMTOFF_IN_TM
+    // Define it for some systems which don't (always) use configure but are
+    // known to have tm_gmtoff field.
+    #if defined(__WXPALMOS__) || defined(__DARWIN__)
+        #define WX_GMTOFF_IN_TM
+    #endif
+#endif
+
+// ============================================================================
+// implementation
+// ============================================================================
+
+// returns the time zone in the C sense, i.e. the difference UTC - local
+// (in seconds)
+int wxGetTimeZone()
+{
+#ifdef WX_GMTOFF_IN_TM
+    // set to true when the timezone is set
+    static bool s_timezoneSet = false;
+    static long gmtoffset = LONG_MAX; // invalid timezone
+
+    // ensure that the timezone variable is set by calling wxLocaltime_r
+    if ( !s_timezoneSet )
+    {
+        // just call wxLocaltime_r() instead of figuring out whether this
+        // system supports tzset(), _tzset() or something else
+        time_t t = time(NULL);
+        struct tm tm;
+
+        wxLocaltime_r(&t, &tm);
+        s_timezoneSet = true;
+
+        // note that GMT offset is the opposite of time zone and so to return
+        // consistent results in both WX_GMTOFF_IN_TM and !WX_GMTOFF_IN_TM
+        // cases we have to negate it
+        gmtoffset = -tm.tm_gmtoff;
+
+        // this function is supposed to return the same value whether DST is
+        // enabled or not, so we need to use an additional offset if DST is on
+        // as tm_gmtoff already does include it
+        if ( tm.tm_isdst )
+            gmtoffset += 3600;
+    }
+    return (int)gmtoffset;
+#elif defined(__DJGPP__) || defined(__WINE__)
+    struct timeb tb;
+    ftime(&tb);
+    return tb.timezone*60;
+#elif defined(__VISUALC__)
+    // We must initialize the time zone information before using it (this will
+    // be done only once internally).
+    _tzset();
+
+    // Starting with VC++ 8 timezone variable is deprecated and is not even
+    // available in some standard library version so use the new function for
+    // accessing it instead.
+    #if wxCHECK_VISUALC_VERSION(8)
+        long t;
+        _get_timezone(&t);
+        return t;
+    #else // VC++ < 8
+        return timezone;
+    #endif
+#elif defined(WX_TIMEZONE) // If WX_TIMEZONE was defined by configure, use it.
+    return WX_TIMEZONE;
+#elif defined(__BORLANDC__) || defined(__MINGW32__) || defined(__VISAGECPP__)
+    return _timezone;
+#elif defined(__MWERKS__)
+    return 28800;
+#else // unknown platform -- assume it has timezone
+    return timezone;
+#endif // WX_GMTOFF_IN_TM/!WX_GMTOFF_IN_TM
+}