Einfach voneinander abziehen:
Delphi-Quellcode:
procedure TForm1.Button1Click(Sender: TObject);
var
Time1, Time2, TimeDiff: TTime;
begin
Time1 := StrToTime(Edit1.Text);
Time2 := StrToTime(Edit2.Text);
TimeDiff := Time1-Time2;
ShowMessage(TimeToStr(TimeDiff));
end;