Interactively, you can right click on a filled date cell and select "fill", and then you can choose to fill to the end of the table, or fill to a specified row.
Or you can run this little script and it will do the job for you
Names Default To Here( 1 );
dt = Current Data Table();
For Each Row(
If( Is Missing( :date ) == 0,
currDate = :date
);
:date = currDate;
);
Jim