diff --git a/rtl/objpas/sysutils/dati.inc b/rtl/objpas/sysutils/dati.inc
index ef187dfb14..2fdf8313dc 100644
--- a/rtl/objpas/sysutils/dati.inc
+++ b/rtl/objpas/sysutils/dati.inc
@@ -216,8 +216,9 @@ end;
 
 procedure DateTimeToSystemTime(DateTime: TDateTime; out SystemTime: TSystemTime);
 begin
-  DecodeDate(DateTime, SystemTime.Year, SystemTime.Month, SystemTime.Day);
+  DecodeDateFully(DateTime, SystemTime.Year, SystemTime.Month, SystemTime.Day,SystemTime.DayOfWeek);
   DecodeTime(DateTime, SystemTime.Hour, SystemTime.Minute, SystemTime.Second, SystemTime.MilliSecond);
+  Dec(SystemTime.DayOfWeek);
 end ;
 
 {   SystemTimeToDateTime converts SystemTime to a TDateTime value   }
diff --git a/rtl/objpas/sysutils/datih.inc b/rtl/objpas/sysutils/datih.inc
index 12f3bf07a5..151b43bcb2 100644
--- a/rtl/objpas/sysutils/datih.inc
+++ b/rtl/objpas/sysutils/datih.inc
@@ -93,7 +93,7 @@ type
 {$if not(defined(windows)) and not(defined(win32))}
    { Win32 reuses the struct from the Windows unit }
    TSystemTime = record
-      Year, Month, Day: word;
+      Year, Month, Day, DayOfWeek: word;
       Hour, Minute, Second, MilliSecond: word;
    end ;
 {$endif windows}