Delphi TTimer providing unusual results in Win 10

Question

I have an app that allows my users to turn on and off a timer to track their time spent on a certain task. The timer runs a clock used to show the elapsed time to the user, much like a stopwatch.

The code below has worked as I thought it should for a few years now. However, when the app is run on Win 10, sometimes the "time" rate speeds up by 2 or 3 times during a session. If the user restarts the app, it may run at normal speed.

Win 10 Delphi 10.3

procedure TfmTimeCard.btnTimerClick(Sender: TObject);
begin
  if btnTimer.Caption = 'Start &Timer' then
  begin
    btnTimer.Down := True;
    btnTimer.Caption := 'Stop &Timer';
    pnlTimer.Color := clPurple;
    btnResume.Enabled := True;
    btnAssign.Enabled := False;
    Timer1.Enabled := true;
    UpdateTimer.Enabled := True;
    ElapsedTime := ElapsedTime;
    //btnPostRecord.Enabled := False;
    btnCancel.Enabled := False;
    btnDeleteTimeCard.Enabled := False;
  end
  else
  begin
    btnTimer.Down := False;
    btnTimer.Caption := 'Start &Timer';
    pnlTimer.ParentColor := True;
    btnResume.Enabled := False;
    btnAssign.Enabled := True;
    pnlTimer.Color := clMoneyGreen;
  end;
end;

procedure TfmTimeCard.Timer1Timer(Sender: TObject);
begin
  if btnTimer.Caption = 'Stop &Timer' then
  begin
    ElapsedTime := ElapsedTime + 0.0000115740;
    cxClock1.time := ElapsedTime;
    cxTimeEditTimer.Time := ElapsedTime;
  end;
end;

Answer 1

This is a terrible way to keep track of elapsed time with a TTimer. TTimer is not a real-time timer, or even an accurate timer. It is based on the WM_TIMER window message, which is

a low-priority message. The GetMessage and PeekMessage functions post this message only when no other higher-priority messages are in the thread's message queue.

Don't calculate your ElapsedTime based on how often the TTimer fires its OnTimer event. Keep track of the current time when starting the TTimer, and then subtract that value from the next current time whenever the OnTimer event is eventually generated. That will give you a more real elapsed time.

Try something more like this:

uses
  ..., System.DateUtils;

private
  StartTime: TDateTime;
  ElapsedSecs: Int64;

procedure TfmTimeCard.btnTimerClick(Sender: TObject);
begin
  if btnTimer.Tag = 0 then
  begin
    btnTimer.Tag := 1;
    ...
    ElapsedSecs := 0;
    StartTime := Now;
    Timer1.Enabled := true;
    ...
  end
  else
  begin
    btnTimer.Tag := 0;
    ...
    ElapsedSecs := SecondsBetween(Now, StartTime);
    Timer1.Enabled := false;
    ...
  end;
end;

procedure TfmTimeCard.Timer1Timer(Sender: TObject);
begin
  if btnTimer.Tag = 1 then
  begin
    ElapsedSecs := SecondsBetween(Now, StartTime);
    // use ElapsedSecs as needed ...
  end;
end;

Or:

uses
  ..., Winapi.Windows;

private
  StartTime: DWORD;
  ElapsedSecs: Integer;

procedure TfmTimeCard.btnTimerClick(Sender: TObject);
begin
  if btnTimer.Tag = 0 then
  begin
    btnTimer.Tag := 1;
    ...
    ElapsedSecs := 0;
    StartTime := GetTickCount;
    Timer1.Enabled := true;
    ...
  end
  else
  begin
    btnTimer.Tag := 0;
    ...
    ElapsedSecs := (GetTickCount - StartTime) div 1000;
    Timer1.Enabled := false;
    ...
  end;
end;

procedure TfmTimeCard.Timer1Timer(Sender: TObject);
begin
  if btnTimer.Tag = 1 then
  begin
    ElapsedSecs := (GetTickCount - StartTime) div 1000;
    // use ElapsedSecs as needed ...
  end;
end;

Or:

uses
  ..., System.Diagnostics;

private
  SW: TStopwatch;
  ElapsedSecs: Integer;

procedure TfmTimeCard.btnTimerClick(Sender: TObject);
begin
  if not SW.IsRunning then
  begin
    ...
    ElapsedSecs := 0;
    SW := TStopWatch.Start;
    Timer1.Enabled := true;
    ...
  end
  else
  begin
    ...
    SW.Stop;
    ElapsedSecs := Trunc(SW.Elapsed.TotalSeconds);
    Timer1.Enabled := false;
    ...
  end;
end;

procedure TfmTimeCard.Timer1Timer(Sender: TObject);
begin
  if SW.IsRunning then
  begin
    ElapsedSecs := Trunc(SW.Elapsed.TotalSeconds);
    // use ElapsedSecs as needed ...
  end;
end;