[NumLock]

Код:

var
  s1, s2, s: String;
begin
  s1:='08:30';
  s2:='20:30';
  s:='10:50';
  if (StrToTime(s)>=StrToTime(s1)) and (StrToTime(s)<=StrToTime(s2)) then
    ShowMessage('ok');  
end;