A guy sits in a pub on a foggy night, goes out, maybe crosses the street, or re-enters the pub and is in a different time. Not a Callahan story.
It might be the Larry Niven story "For a Foggy Night":
In one of Niven's rare stories of cross-dimensional travel, an unnamed protagonist discovers the real reason that things seem so foggy on a foggy night.
The story does start in a bar, and the narrator gets lost in the fog and ends up in an alternate universe. It's not time travel, though, it's travel between alternative universes.