Restar 2 horas...

[roman]

Puedes usar las funciones MinutesBetween o SecondsBetween de la unidad DateUtils:

var
  Time1, Time2: TTime;
  Minutes, Seconds: Int64;

begin
  Time1 := StrToTime(Edit1.Text);
  Time2 := StrToTime(Edit2.Text);

  Minutes := MinutesBetween(Time1, Time2);
  Seconds := SecondsBetween(Time1, Time2);
end;

// Saludos