function GetSubDateTime(S1, S2:string): string;
var
  T1, T2: TDateTime;
  D, H, M, S: Integer;
  Value: Int64;
begin
  T1 := StrToDateTime(S1);
  T2 := StrToDateTime(S2);
  Value := SecondsBetween(T1, T2);
  D := Value div SecsPerDay;
  H := Value mod SecsPerDay div SecsPerHour;
  M := Value mod SecsPerDay mod SecsPerHour div SecsPerMin;
  S := Value mod SecsPerDay mod SecsPerHour mod SecsPerMin;
  result := Format('%.2d天 %.2d:%.2d:%.2d',[D, H, M, S]);
end;

调用：
var
Caption: string;
begin
  Caption := GetSubDateTime(S1, S2);
  memo1.liens.add(Caption);
end;

procedure TForm2.Timer1Timer(Sender: TObject);
var
  S1, S2: string;
begin
  S1 := FormatDateTime('yyyy/mm/dd hh:mm:ss', now());   // 我用的XE，所以提前出来的系统时间是这种格式
  S2 := '2015/9/22 01:02:03';                           // 这里时间要和获取到的系统时间一致
  GetSubDateTime(S1, S2);
  Memo1.Lines.Add(GetSubDateTime(S1, S2));
end;