Optional Parameter Not Working When Trying To Pass Null Value To Database?

I am hardcoding an insert statement to a database, based on certain details a user enters into a windows form. The form has a few textbox and datetimepicker controls for the purpose. This form enters a single row in a single table in the database.The table has a few possible null columns defined, in those cases where the user does not have any data for those columns at that particular time.

I have to provide controls for passing data to those null columns nevertheless. In my case, I have two such columns that accept null values, and two corresponding controls - a textbox and a datetimepicker. The datetimepicker has a checkbox added to it so that if it is supposed to be null it doesn't pass the default current date value, and remains unchecked to explicitly specify that it is supposed to be null. By default it is unchecked.

My code is structured as follows:

Form1:
'the sub referenced here resides in a dll file in my dataaccesslayer code, and in my actual example is referenced properly.
'I have shortened the code here for simplicity

[code]....

Now it gives me an error whenever the code comes to the executenonquery(), and says something to the tune of "SqlDateTimeOverflow. Must be between 1/1/1753 12:00:00 AM and 12/31/9999 11:59:59 PM."

I think the value of 'Nothing' from the subprocedure's paramter is not being assigned to the sqlcommand's parameter as a database NULL, which is what I need to pass to the database.I know instead of using optional parameters I can easily create overloaded subprocedures where one sub allows not passing values where the column(s) is supposed to null and other overloaded subs permit combinations of which column(s) are null or not. But it is simple as long as there is just one, or at the most two null columns in a table. If I have dozens of null values in a table that the user can choose to enter or not in a data entry form?