Don't use localtime_r on Windows

That code was not tested actually.
On windows struct tm doesn't have localtime_r and tm_zone

src/celestia/qt/qtappwin.cpp

@@ -57,7 +57,6 @@
 //#include "qtvideocapturedialog.h"
 #include "celestia/scriptmenu.h"
 #include "celestia/url.h"
-#include "celutil/localtime.h"
 #include "qtbookmark.h"
 
 #if defined(_WIN32)
@@ -1336,6 +1335,7 @@ void CelestiaAppWindow::createMenus()
     m_appCore->getSimulation()->setSyncTime(check);
 
     // Set up the default time zone name and offset from UTC
+#ifndef _WIN32
     time_t curtime = time(nullptr);
     m_appCore->start(astro::UTCtoTDB((double) curtime / 86400.0 + (double) astro::Date(1970, 1, 1)));
 
@@ -1345,6 +1345,39 @@ void CelestiaAppWindow::createMenus()
         m_appCore->setTimeZoneBias(result.tm_gmtoff);
         m_appCore->setTimeZoneName(result.tm_zone);
     }
+#else
+    TIME_ZONE_INFORMATION tzi;
+    DWORD dst = GetTimeZoneInformation(&tzi);
+    if (dst != TIME_ZONE_ID_INVALID)
+    {
+        LONG dstBias = 0;
+        WCHAR* tzName = nullptr;
+
+        if (dst == TIME_ZONE_ID_STANDARD)
+        {
+            dstBias = tzi.StandardBias;
+            tzName = tzi.StandardName;
+        }
+        else if (dst == TIME_ZONE_ID_DAYLIGHT)
+        {
+            dstBias = tzi.DaylightBias;
+            tzName = tzi.DaylightName;
+        }
+
+        if (tzName == nullptr)
+        {
+            m_appCore->setTimeZoneName("   ");
+        }
+        else
+        {
+            char tz_name_out[20];
+            size_t length = wcstombs(tz_name_out, tzName, sizeof(tz_name_out)-1);
+            tz_name_out[std::min(sizeof(tz_name_out)-1, length)] = '\0';
+            m_appCore->setTimeZoneName(tz_name_out);
+        }
+        m_appCore->setTimeZoneBias((tzi.Bias + dstBias) * -60);
+    }
+#endif
 
     // If LocalTime is set to false, set the time zone bias to zero.
     if (settings.contains("LocalTime"))

src/celutil/localtime.h

@@ -1,24 +0,0 @@
-#ifndef LOCALTIME_H
-#define LOCALTIME_H
-
-#ifdef _WIN32
-/*
-  Very dump implementation of localtime_r for Windows platform.
-*/
-extern "C" {
-    struct tm *localtime_r(time_t *clock, struct tm *result)
-    {
-        struct tm *p = localtime(clock);
-
-        if (p)
-        {
-            *result = *p;
-            return result;
-        }
-
-        return NULL;
-    }
-}
-#endif
-
-#endif