Filmed in May 1969 (based on onscreen evidence)
Sometimes you can tell when a movie was filmed based on magazines and newspapers seen on the screen. When the Kellermans are entering the airport, they stop at the news stand, and purchase a magazine. If you look closely, on the magazine rack (at the top on the right of said rack) is an issue of LIFE with a yellow cover. Upon close examination, it can be identified as the May 16, 1969 issue entitled "Collision Course In High Schools."
Check the link below and see if you can spot the LIFe in the scene at the airport!!
http://www.2neatmagazines.com/life/1969cover.html