Function SetFileDate(
  Const FileName : String;
  Const FileDate : TDateTime): Boolean;
var
 FileHandle        : THandle;
 FileSetDateResult : Integer;
begin
 try
  try
   FileHandle := FileOpen
      (FileName,
       fmOpenWrite OR fmShareDenyNone);
   if FileHandle > 0 Then  begin
    FileSetDateResult :=
      FileSetDate(
        FileHandle,
        DateTimeToFileDate(FileDate));
      result := (FileSetDateResult = 0);
    end;
  except
   Result := False;
  end;
 finally
  FileClose (FileHandle);
 end;
end;

{Использование:}
SetFileDate('c:\mydir\myfile.ext', Now)