Aye but we have no indication that NOTLD is set in 1968 specifically (beyond the superficial details).
The Jon Pertwee Doctor Who episodes were meant to be set about six years in the future of the time of transmission but now we can clearly see that The Time Warrior wasn't filmed in 1980 (so much so that annoyingly these were either retconned or mistaken to have taken place in the early seventies by later writers and producers which has caused much a do in Doctor Who fan circles who love to argue about these things).
Also to make Dawn Of The Dead with late sixties details would have pushed the already tight budget to breaking point and robbed the film of it's cost saving and satirically ripe central location.
The mall in the film was only just built and such buildings were practically non existent when NOTD was filmed.
That's why I said we are supposed to use our imaginations when viewing these films.
The aren't set in a specific year but like Max Headroom they are set '20 minutes in the future'.
They follow on from each other pretty much in the manner I described but they begin any minute now (whenever that is).