Ok, I did more testing. Thank you for the feedback @arnan
I was only using Safari Version 11.1 before, and all the dates were being displayed as “06-06-2018”.
But when I switched to Chrome Version 67.0.3393.4 for example and created a new advert and added a schedule while creating an advert, it instead shows yyyy/mm/dd like “2018/06/06” and it SUCCESSFULLY created the correct schedule.
But… in Chrome when I then try to edit the Schedule (because I want to check the box to delete it a day after the schedule finishes) and save that change, it reverts to the 1970 date.
So it seems partially a browser issue and partially a server-side issue?