I assume you mean that you want to write null if the date is same as default.
This is one way to do it.
DateTime defaultDate = DateTime.Parse("01/01/1753");
DateTime someDate1 = new DateTime();
if (someDate1.Date == defaultDate.Date)
Console.WriteLine("null");
else
Console.WriteLine(someDate1.ToString());
DateTime someDate2 = DateTime.Parse("01/01/1753");
if (someDate2.Date == defaultDate.Date)
Console.WriteLine("null");
else
Console.WriteLine(someDate2.ToString());