Advice which component to use

Use a TDateTimePicker as a control to pick a time. You can set the Kind property in the Object Inspector to Time.

Set your TTimer's interval to the number of seconds between each check, for example 1000 for 1 second. (Technically you might decide to check only every 5 seconds, or 15 seconds, etc).

Then double-click on the timer to edit its OnTimer event, and say something like this

uses
DateUtils <--- add dateutils to the uses clause of your unit

begin
  If secondsbetween(DateTimePicker1.Time, now) < 5 then
  begin
   Timer1.enabled := false;
   Showmessage('Alarm!');
  end;
end;

I would use anotehjr approach (the component is fine).

I would calculate the seconds from "now" (the now function) and all the 3 datetimepickers and maybe store them into an array;

then get the minimum value, set the timer interval to that minimum value, remove the min value from the list and start the timer. when timer fires, do the abopve operation again until the list is empty.

it's much more optimal then making the timer fire at every x seconds :)

This is what i got but it doesn't work:

procedure TMainForm.DateTimePicker1Change(Sender: TObject);
begin
  AlarmDateTime := DateTimePicker1.Time
end;
(*----------------------------------------------*)
procedure TMainForm.StartBtnClick(Sender: TObject);
begin
    Timer1.enabled := True;
end;
(*----------------------------------------------*)
procedure TMainForm.Timer1Timer(Sender: TObject);
begin
  if SecondsBetween(AlarmDateTime, Now) < 5 then
   begin
   Timer1.enabled := false;
   Showmessage('Alarm!');
   end;
end;
(*----------------------------------------------*)

P.

What is not working?
You can do this for debugging:

procedure TMainForm.Timer1Timer(Sender: TObject);
begin
  MainForm.Caption := inttostr(SecondsBetween(AlarmDateTime, Now);
  if SecondsBetween(AlarmDateTime, Now) < 5 then
   begin
   Timer1.enabled := false;
   Showmessage('Alarm!');
   end;
end;
(*----------------------------------------------*)

I am testing it, and testing it but i still don't know why it isn't working!!!

P.

When i set the timer at 15:00 and when the system timer is 15:00 also
the counter in the caption of the mainform stop counting for a couple of seconds
and the continues. But nothing happens. My showmessage doesn't work or a beep.

P.

timer interval = 1000

It works now.

Thank you

Peter K.