FormatDateTime in Delphi 12

[Oscar Hernández 0]

Hello everyone,

I am working with Delphi 12 and using some functions to convert TDateTime to String and vice versa. However, while testing the FormatDateTime function, I noticed an issue. When I use the format dd/mm/yyyy, specifying the slash (/) as the date separator, but my system's short date format is set to aaaa-MM-dd (using a hyphen as the separator), the output is in dd-mm-yyyy format instead of the expected dd/mm/yyyy.

Is this a bug, or is there a specific reason for this behavior?

Here is a snapshot of my short date format and the result of the example above.

 

Edited March 26 by Oscar Hernández
I have updated the image to use the TDateTime variable.

[Remy Lebeau 1572]

You are using the version of FormatDateTime() that relies on global variables initialized with your PC's locale settings. For instance, the '/' specifier is not a literal slash character, it is a placeholder that uses the global DateSeparator variable in the SysUtils unit. This is documented behavior.
 

You can update those globals to customize things like the date/time separators, but this affects the whole app, and is not thread-safe. When you need custom formatting that is locale-agnostic, you should use the version of FormatDateTime() that takes a TFormatSettings parameter, eg:

procedure TForm1.Button1Click(Sender: TObject);
var
  lDate: TDateTime;
  lFmt: TFormatSettings;
begin
  lFmt := TFormatSettings.Create;
  lFmt.DateSeparator := '/';
  lDate := EncodeDate(2025, 3, 26);
  Edit1.Text := FormatDateTime('dd/mm/yyyy', lDate, lFmt);
end;

The alternative is to wrap desired literal text with quotes in your format string, eg:

procedure TForm1.Button1Click(Sender: TObject);
var
  lDate: TDateTime;
begin
  lDate := EncodeDate(2025, 3, 26);
  Edit1.Text := FormatDateTime('dd"/"mm"/"yyyy', lDate);
end;

Edited March 27 by Remy Lebeau