How to correct / shift subtitle time in SRT (SubRip) files?

Question

How to correct/shift subtitle time forward and backward? Subtitle time format looks like this: 00:00:52,656 --> 00:00:56,326

If subtitle and audio aren't synchronized, for example, subtitle shows up before voice/audio, then all times of subtitle lines (time format: 00:00:52,656 --> 00:00:56,326) should be corrected.

So, if time of all subtitle lines must be changed/shifted for 2 sec. forward, then, this time for subtitle line: 00:00:52,656 --> 00:00:56,326 should be changed to: 00:00:54,656 --> 00:00:58,326.

And this refers to all times in the subtitle file, not just one line of text/one time.

---

Example of how SubRip (.srt) file looks like:

1
00:00:52,656 --> 00:00:56,326
Kanalska Zona: Panama

2
00:00:56,335 --> 00:00:59,755
Francuzi su pokušali da izgrade
kanal pre Amerikanaca.

Answer 1

Providing that the format of each line in your input is always 00:00:00,000 --> 00:00:00,000, then this routine will convert the string times to TDateTime, add or subtract the shift, and rewrite the line:

procedure ShiftSubtitleTimes(Lines: TStrings; Diff: TTime);
var
  FS: TFormatSettings;
  I: Integer;
  T1: TDateTime;
  T2: TDateTime;
begin
  // Ensure using the correct time separator
  FS.TimeSeparator := ':';
  // Parse each line separately
  for I := 0 to Lines.Count - 1 do
  begin
    // Convert the two time strings to time values
    if not TryStrToTime(Copy(Lines[I], 1, 8), T1, FS) then
      // But skip line in case of wrong format
      Continue;
    T1 := T1 + StrToInt(Copy(Lines[I], 10, 3)) / MSecsPerDay;
    T2 := StrToTime(Copy(Lines[I], 18, 8), FS);
    T2 := T2 + StrToInt(Copy(Lines[I], 27, 3)) / MSecsPerDay;
    // Add the shift
    T1 := T1 + Diff;
    T2 := T2 + Diff;
    // Rewrite the line
    Lines[I] := FormatDateTime('hh:nn:ss,zzz --> ', T1, FS) +
      FormatDateTime('hh:nn:ss,zzz', T2, FS);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  List: TStringList;
begin
  List := TStringList.Create;
  try
    List.LoadFromFile('Filename.dat');
    Memo1.Lines.Add('Input:');
    Memo1.Lines.AddStrings(List);
    Memo1.Lines.Add('');
    // Shift 3,5 seconds backwards:
    ShiftSubtitleTimes(List, -3.5 / SecsPerDay);  
    Memo1.Lines.Add('Output:');
    Memo1.Lines.AddStrings(List);
  finally
    List.Free;
  end;
end;

Edit:

Due to your edit, now input may contain 'wrong' lines that need no conversion too.