Ex-publican here. Confidently ignore expiry dates on bottles so long as the caps are properly intact. I once drank several that were nearly three years out of date with no ill effect or much difference in taste.
When it comes to draught though, it's an entirely different matter. If I had some lager that had gone past its date, I would use tactics to try and shift it quickly i.e. just pretend we'd run out of the alternatives one evening, then the next morning say we'd just had a delivery and they're all back on.
I personally never got into the position, because draught lager moves fast in general and because I was damn good at my job, but if I'd ever got to four or five days past the date I would have written it off. A lot of people can taste the difference, but more importantly because once poured there is a completely dodgy smell to it, no matter how clean the lines are. Draught isn't a 100% closed system like bottled lagers, and that's why time is relevant.
Never sold cans, so no comment there.