How do I convert milliseconds to a TDateTime?

Question

I'm doing a long loop downloading thousands of files. I would like to display an estimated time remaining, since it could take hours. However, with what I've written, I get an average number of milliseconds. How do I convert this average download time from milliseconds to a TDateTime?

See where I'm setting Label1.Caption:

procedure DoWork;
const
  AVG_BASE = 20; //recent files to record for average, could be tweaked
var
  Avg: TStringList; //for calculating average
  X, Y: Integer; //loop iterators
  TS, TE: DWORD; //tick counts
  A: Integer; //for calculating average
begin
  Avg:= TStringList.Create;
  try
    for X:= 0 to FilesToDownload.Count - 1 do begin //iterate through downloads
      if FStopDownload then Break; //for cancelling
      if Avg.Count >= AVG_BASE then //if list count is 20
        Avg.Delete(0); //remove the oldest average
      TS:= GetTickCount; //get time started
      try
        DownloadTheFile(X); //actual file download process
      finally
        TE:= GetTickCount - TS; //get time elapsed
      end;
      Avg.Add(IntToStr(TE)); //add download time to average list
      A:= 0; //reset average to 0
      for Y:= 0 to Avg.Count - 1 do //iterate through average list
        A:= A + StrToIntDef(Avg[Y], 0); //add to total download time
      A:= A div Avg.Count; //divide count to get average download time
      Label1.Caption:= IntToStr(A); //<-- How to convert to TDateTime?
    end;
  finally
    Avg.Free;
  end;
end;

PS - I'm open to different ways of calculating the average speed of the last 20 (or AVG_BASE) downloads, because I'm sure my string list solution isn't the best. I don't want to calculate it based on all downloads, because speed may change over that time. Therefore, I'm just checking the last 20.

Answer 1

A TDateTime value is essentially a double, where the integer part is the number of days and fraction is the time.

In a day there are 24*60*60 = 86400 seconds (SecsPerDay constant declared in SysUtils) so to get A as TDateTime do:

dt := A/(SecsPerDay*1000.0); // A is the number of milliseconds 

A better way to clock the time would be to use the TStopWatch construct in the unit Diagnostics.

Example:

sw.Create;
.. 
sw.Start;
// Do something
sw.Stop;
A := sw.ElapsedMilliSeconds;
// or as RRUZ suggested ts := sw.Elapsed; to get the TimeSpan 

To get your average time, consider using this moving average record:

Type
  TMovingAverage = record
  private
    FData: array of integer;
    FSum: integer;
    FCurrentAverage: integer;
    FAddIx: integer;
    FAddedValues: integer;
  public
    constructor Create(length: integer);
    procedure Add( newValue: integer);
    function Average : Integer;
  end;

procedure TMovingAverage.Add(newValue: integer);
var i : integer;
begin
  FSum := FSum + newValue - FData[FAddIx];
  FData[FAddIx] := newValue;
  FAddIx := (FAddIx + 1) mod Length(FData);
  if (FAddedValues < Length(FData)) then
    Inc(FAddedValues);
  FCurrentAverage := FSum div FAddedValues;
end;

function TMovingAverage.Average: Integer;
begin
  Result := FCurrentAverage;
end;

constructor TMovingAverage.Create(length: integer);
var
  i : integer;
begin
  SetLength( FData,length);
  for i := 0 to length - 1 do
    FData[i] := 0;
  FSum := 0;
  FCurrentAverage := 0;
  FAddIx := 0;
  FAddedValues := 0;
end;