DateTime does not have a "format" - it is a datetime.
It is only when you are trying to display a datetime that a "format" becomes visible, and that will mostly depend on where you are in the world, or which Culture you explicitly use.
You can use the .ToString() to format your output e.g.
DateTime _tempDate = DateTime.MinValue;
_tempDate = Convert.ToDateTime(dr[6]);
_tempDate = _tempDate.AddMonths(1);
string date = _tempDate.ToString("yyyy/MM/dd hh:mm:ss tt");
Console.WriteLine(date);
Although personally I would not use Convert.ToDateTime but TryParse or TryParseExact.
There are several ways that you can force the format of the output string - see
Custom date and time format strings | Microsoft Docs[
^]