If you are inserting the row there are two ways:
INSERT into my table (Col1,col2,ColDate) values (1,2,NULL)
INSERT into mytable (col1,col2) values (1,2)
First one explicitly sets it to null while the second implicitly sets it to null (if no default value has been specified for the date column).
For updating, you must explicitly set it to NULL.
HTH
Andy