Valid Input Syntax for Type DATE

I'm trying to insert rows into a table, but some date and time columns are
missing values. In the INSERT INTO ... statements of the .sql file I've
tried various formats: ,, and ,'', and ,' ', but they all generate the error
of invalid syntax for type date.

When I have missing date of the date and time types, how do I validly
represent them in the input file?

Rich