I'm using version 2016.1.226 and the new "Is null" and "Is not null" filters are not working correctly on DateTime columns (as well as DateTime? columns).
When I select "Is null" on a DateTime column, it seems to filter the column but the Cancel button (the X button next to filter) does not appear. Then I select "Is not null" and then "Is null" again, the Cancel button appears but it does not do anything. So the only way to get rid of the filtering is to refresh the page.
These filter operators seem to work on String columns though.