Xsd Files And Null Datetime Columns

Ive created a Data Access Layer (xsd file) and some of the tables have DateTime columns.  I need to allow NULL values to be saved, but when I try to do that I get an error

In my code (c#) ive used Nullable DateTime, but when I try to assign a nullable Datetime to a DateTime column, I get this error

cannot implicitly convert type 'System.DateTime?' to 'System.DateTime'

I understand the reasons why, so how do I save null DateTime values to the underlying database through a schema ?

In the schema file, the DateTime columns have <DBNULL> as the default value, AllowBDNull is true, but I am only allowed 'Throw exception' on the NULL value property

I could probably do this another way (by not using a schema and writing the code myself)  but Id prefer to keep the strongly typed code.

 I dont want to use MAx or MIn values for the default datetime values, so how can this be done ?