diff --git a/extra/calendar/windows/windows.factor b/extra/calendar/windows/windows.factor
index 9e34fdac00..acbae2fcd3 100755
--- a/extra/calendar/windows/windows.factor
+++ b/extra/calendar/windows/windows.factor
@@ -8,8 +8,14 @@ T{ windows-calendar } calendar-backend set-global
 
 : TIME_ZONE_ID_INVALID HEX: ffffffff ; inline
 
-M: windows-calendar gmt-offset ( -- float )
+M: windows-calendar gmt-offset ( -- hours minutes seconds )
     "TIME_ZONE_INFORMATION" <c-object>
-    dup GetTimeZoneInformation
-    TIME_ZONE_ID_INVALID = [ win32-error ] when
-    TIME_ZONE_INFORMATION-Bias 60 / neg ;
+    dup GetTimeZoneInformation {
+        { [ dup TIME_ZONE_ID_INVALID = ] [ win32-error ] }
+        { [ dup { TIME_ZONE_ID_UNKNOWN TIME_ZONE_ID_STANDARD } member? ]
+            [ TIME_ZONE_INFORMATION-Bias 60 / neg ] }
+        { [ dup TIME_ZONE_ID_DAYLIGHT = ] [
+            [ TIME_ZONE_INFORMATION-Bias 60 / neg ]
+            [ TIME_ZONE_INFORMATION-DaylightBias ] bi
+        ] }
+    } cond ;