procedure TForm3.Button1Click(Sender: TObject);
var
  MySettings: TFormatSettings;
  s: string;
  d: TDateTime;
begin
  GetLocaleFormatSettings(GetUserDefaultLCID, MySettings);
  MySettings.DateSeparator := '-';
  MySettings.TimeSeparator := ':';
  MySettings.ShortDateFormat := 'mm-dd-yyyy';
  MySettings.ShortTimeFormat := 'hh:nn:ss';

  s := DateTimeToStr(Now, MySettings);
  ShowMessage(s);
  d := StrToDateTime(s, MySettings);
  ShowMessage(DateTimeToStr(d, MySettings));
end;

However, if you used 'mmm-dd-yyyy' as the ShortDateFormat in the example above (to display Jan-09-2006), you would get an error when calling StrToDateTime on this string. The workaround is to call d := VarToDateTime(s) instead, since that function supports month names when encoding the TDateTime. I logged this bug as QC 23301, so if you'd like to see this fixed, please rate this entry and vote on it if it's important enough to you.