How to format date in MM/DD/YYYY in Delphi

TryStrToDate is a Delphi library function that will try to convert a string into a TDateTime type. In my code there were some formats that I wanted to accept that are not allowed by the TryStrToDate function. To convert those dates if the first method fails I call VarToDateTime. That calls a variant function that supports additional formats.

As Remy commented there is an option parameter of a TFormatSettings type that allows you some measure of control over the formats accepted. I still needed to use VarToDateTime in order to covert dates that spelled out the month instead of using numeric values.

var
  DateValue: TDateTime;

for i:= 1 to ParamCount do
begin
  // If we don't have a valid date see if it looks like a different
  // normal date field.
  if TryStrToDate(Param[i], DateValue) = false then
    continue; 

  // If the simple TryStrToDate does not work use the Variants function,
  // there is no "Try" version so catch the error here.
  try
    DateValue := VarToDateTime(Param[i]);
  except
    continue;
  end;

  // Use/Reformat DateValue here
 Param[i] := FormatDateTime('MM/DD/YYYY', DateValue);
end;

Additional Information on using TFormatSettings
The TFormatSettings parameter can change if a date is interpreted to have the days or the month listed first. If you do not specify a Format Setting you will be using the Operating System Default. TryStrToDate looks at the short date format string to determine what order to expect. It looks at the string format for the first matching letter to decide on acceptable formats:

case Chr(Ord(DateFormat[I]) and $DF) of
  'E': Result := doYMD;
  'Y': Result := doYMD;
  'M': Result := doMDY;
  'D': Result := doDMY;

Here is a test procedure that can show you the difference. You can create a FormatSettings based on a locale, or you can set the short date format directly.

procedure TForm7.Button3Click(Sender: TObject);
var
  s: string;
  d: TDateTime;
  FormatUS: TFormatSettings;
  FormatGB: TFormatSettings;
begin
  s := '5/10/2017';

  Memo1.Lines.Append('Testing GB');
  FormatGB := TFormatSettings.Create('en-GB');
  if TryStrToDate(s, d, FormatGB) = false then
  begin
    Memo1.Lines.Append(s + ' is not a valid date');
  end
  else
  begin
    Memo1.Lines.Append('Found Date: ' + FormatDateTime('dd MMMM YYYY', d));
    // will print: Found Date: 05 October 2017
  end;

  Memo1.Lines.Append('');
  Memo1.Lines.Append('Testing US');
  FormatUS := TFormatSettings.Create('en-US');
  if TryStrToDate(s, d, FormatUS) = false then
  begin
    Memo1.Lines.Append(s + ' is not a valid date');
  end
  else
  begin
    Memo1.Lines.Append('Found Date: ' + FormatDateTime('dd MMMM YYYY', d));
  end;

  Memo1.Lines.Append('');
  Memo1.Lines.Append('Testing with modified ShortDate Format');
  FormatUS.ShortDateFormat := 'yyyy/mm/dd';
  if TryStrToDate(s, d, FormatUS) = false then
  begin
    Memo1.Lines.Append(s + ' is not a valid date');
  end
  else
  begin
    Memo1.Lines.Append('Found Date: ' + FormatDateTime('dd MMMM YYYY', d));
  end;

end;

The output will show:

Testing GB
Found Date: 05 October 2017

Testing US
Found Date: 10 May 2017

Testing with modified ShortDate Format
5/10/2017 is not a valid date