uses
   Registry;

 // Write TDateTime to Registry 
procedure Reg_WriteDateTime(dwRootKey: DWord; const sKey: string; const sField: string; aDate: TDateTime);
 begin
   with TRegistry.Create do
   try
     RootKey := dwRootKey;
     if OpenKey(sKey, True) then
     begin
       try
         WriteBinaryData(sField, aDate, SizeOf(aDate));
       finally
         CloseKey;
       end;
     end;
   finally
     Free;
   end;
 end;

 // Read TDateTime from Registry 
function Reg_ReadDateTime(dwRootKey: DWord; const sKey: string; const sField: string) : TDateTime;
 begin
   Result := 0; // default Return value 
  with TRegistry.Create do
   begin
     RootKey := dwRootKey;
     if OpenKey(sKey, False) then
     begin
       try
         ReadBinaryData(sField, Result, SizeOf(Result));
       finally
         CloseKey;
       end;
     end;
     Free;
   end;
 end;

 // Example: 

// Write DateTimePicker1's DateTime to Registry 
procedure TForm1.Button1Click(Sender: TObject);
 begin
    Reg_WriteDateTime(HKEY_CURRENT_USER, 'Software\TestXYZ\','DateTime',DateTimePicker1.DateTime);
 end;

 // Set DateTimePicker1's DateTime from Registry 
procedure TForm1.Button2Click(Sender: TObject);
 var
  ATime: TDateTime;
 begin
   ATime := Reg_ReadDateTime(HKEY_CURRENT_USER, 'Software\TestXYZ\','DateTime');
   if ATime <> 0 then
     DateTimePicker1.DateTime := TDateTime(ATime);
 end;