Explicitly disambiguate local time zone from UTC

Don't rely on time zone offset to check whether it is local as this
doesn't, and can't, work for the local time zone in Great Britain which
uses the same offset as UTC, but does use DST, unlike the latter.

Add a unit test (albeit disabled by default) checking that the code that
previously didn't work correctly in BST does work now (run the tests
using "TZ=Europe/London ./test wxDateTime-BST-bugs" under Unix to test).

Closes #14317, #17220.

See #10445.

include/wx/datetime.h

@@ -306,7 +306,9 @@ public:
             return tz;
         }
 
-        long GetOffset() const { return m_offset; }
+        bool IsLocal() const { return m_offset == -1; }
+
+        long GetOffset() const;
 
     private:
         // offset for this timezone from GMT in seconds

interface/wx/datetime.h

@@ -254,6 +254,17 @@ public:
         /// Create a time zone with the given offset in seconds.
         static TimeZone Make(long offset);
 
+        /**
+            Return true if this is the local time zone.
+
+            This method can be useful for distinguishing between UTC time zone
+            and local time zone in Great Britain, which use the same offset as
+            UTC (i.e. 0), but do use DST.
+
+            @since 3.1.1
+         */
+        bool IsLocal() const;
+
         /// Return the offset of this time zone from UTC, in seconds.
         long GetOffset() const;
     };

src/common/datetime.cpp

@@ -456,9 +456,8 @@ wxDateTime::TimeZone::TimeZone(wxDateTime::TZ tz)
     switch ( tz )
     {
         case wxDateTime::Local:
-            // get the offset from C RTL: it returns the difference GMT-local
-            // while we want to have the offset _from_ GMT, hence the '-'
-            m_offset = -wxGetTimeZone();
+            // Use a special value for local time zone.
+            m_offset = -1;
             break;
 
         case wxDateTime::GMT_12:
@@ -503,6 +502,13 @@ wxDateTime::TimeZone::TimeZone(wxDateTime::TZ tz)
     }
 }
 
+long wxDateTime::TimeZone::GetOffset() const
+{
+    // get the offset from C RTL: it returns the difference GMT-local
+    // while we want to have the offset _from_ GMT, hence the '-'
+    return m_offset == -1 ? -wxGetTimeZone() : m_offset;
+}
+
 // ----------------------------------------------------------------------------
 // static functions
 // ----------------------------------------------------------------------------
@@ -1432,7 +1438,7 @@ unsigned long wxDateTime::GetAsDOS() const
 
 const tm* wxTryGetTm(tm& tmstruct, time_t t, const wxDateTime::TimeZone& tz)
 {
-    if ( tz.GetOffset() == -wxGetTimeZone() )
+    if ( tz.IsLocal() )
     {
         // we are working with local time
         return wxLocaltime_r(&t, &tmstruct);

tests/datetime/datetimetest.cpp

@@ -683,7 +683,7 @@ void DateTimeTestCase::TestTimeFormat()
 
     const long timeZonesOffsets[] =
     {
-        wxDateTime::TimeZone(wxDateTime::Local).GetOffset(),
+        -1, // This is pseudo-offset used for local time zone
 
         // Fictitious TimeZone offsets to ensure time zone formating and
         // interpretation works
@@ -1638,4 +1638,27 @@ TEST_CASE("wxDateTime::SetOnDST", "[datetime][dst]")
     }
 }
 
+// Tests random problems that used to appear in BST time zone during DST.
+// This test is disabled by default as it only passes in BST time zone, due to
+// the times hard-coded in it.
+TEST_CASE("wxDateTime-BST-bugs", "[datetime][dst][BST][.]")
+{
+    SECTION("bug-17220")
+    {
+        wxDateTime dt;
+        dt.Set(22, wxDateTime::Oct, 2015, 10, 10, 10, 10);
+        REQUIRE( dt.IsDST() );
+
+        CHECK( dt.GetTm().hour == 10 );
+        CHECK( dt.GetTm(wxDateTime::UTC).hour == 9 );
+
+        CHECK( dt.Format("%Y-%m-%d %H:%M:%S", wxDateTime::Local ) == "2015-10-22 10:10:10" );
+        CHECK( dt.Format("%Y-%m-%d %H:%M:%S", wxDateTime::UTC   ) == "2015-10-22 09:10:10" );
+
+        dt.MakeFromUTC();
+        CHECK( dt.Format("%Y-%m-%d %H:%M:%S", wxDateTime::Local ) == "2015-10-22 11:10:10" );
+        CHECK( dt.Format("%Y-%m-%d %H:%M:%S", wxDateTime::UTC   ) == "2015-10-22 10:10:10" );
+    }
+}
+
 #endif // wxUSE_DATETIME