I hardly remember anything! It's an old movie or can be a part of a series from a 90'.
It takes place in a big seaside city, maybe Los Angeles or San Francisco. Portals open to the apocalyptic future, where the sun began to fade. The protagonist gets across one and meets a couple of human survivors who tell him what's happened. Some sort of zombie mutants began to appear, typical human-hunting distorted monsters in rags. They where humanoids and looked like a zombie and mummy combined with bandages. I think maybe there was some environmental protection message, the plan was to solve the pollution problem in the past and change the future.(?)
Maybe because of the strange portals his friends traped in an office in the past and worried for him.(?) The protagonist takes an American football, writes something on it and throws it into a smaller portal. Then in the past, its flying through a curtain of the office window and hits a guy on the head.
In the end the man fights the biggest ugliest zombie monster and go back home in the portal.
I don't know why, but for some reason I remember David Hasselhoff in it. Not the Sliders, Doctor Who, Primeval, Time Cop, Sunshine, The time machine, Morlocks.