SqlDateTime to DateTime

My coworker was having fun working with SqlDateTime and DateTime
structures and he asked me just WTF they were doing in
http://msdn.microsoft.com/en-us/library/aa326475(VS.71).aspx He was
attempting to convert a SqlDateTime into a DateTime and the syntax
just was not clicking. Didn't for me either until I read up on what
the "explicit" meant.
http://msdn.microsoft.com/en-us/library/xhbhezf4(VS.71).aspx

What's all the hubbub? SqlDateTime allows for nulls, DateTime does
not. They're both value types but SqlDateTime implements INullable so
it gets away with it. The $64,000 question then became what happens
when you assign a "null" SqlDateTime to a DateTime and the answer is
it converts it to a sentinal value of 1900-01-01T00:00:00.000

public static void ConvertDates()
{
    // create a sql datetime object with today's date
    System.Data.SqlTypes.SqlDateTime sdt = new System.Data.SqlTypes.SqlDateTime(2009, 11, 18);

    // Re-assign the type as null
    sdt = System.Data.SqlTypes.SqlDateTime.Null;

    // Create a datetime variable and set value to max (9999-12-31T23:59:59)
    DateTime dt = DateTime.MaxValue;

    // A null SqlDateTime is converted to the following datetime {1/1/1900 12:00:00 AM}
    dt = (DateTime)sdt;
}

I wonder whether using your own sentinal value wouldn't be better. As
the above is the minimum date/time value in SQL Server for a datetime,
one could assume that a DateTime (.NET) value of less than the minimum
means NULL. And it's days like today that make me happy and slightly
regretful that I don't deal as deeply in the C# world here at work.