It seems like that Denny's must have opened in 1971? The oldest ads I can find about them hiring cooks are from spring 1971.
July 17, 1964
City to Pay $50,000 For Land
Morrison Motors Ltd. will be paid $50,000 for an 83-foot wide strip of property on Douglas Street, through which the city will extend Finlayson from Douglas to Burnside...
Negotiations have been underway for weeks following the acquisition of about six acres of land on Douglas by the automobile company and the subsequent council decision that Finlayson would have to be extended through the property.
City manager Dennis Young told the committee on Thursday that the asking price of $50,000 was reasonable enough and would spare the city costly expropriation proceedings.