Well, I imagine that we are not robots and if we miss a feed by a couple of minutes or half an hour our supply will be fine. We can be flexible as our bodies are amazing to adjust to the need. Even if you skip a feed/pumping once in a while nothing serious will happen, yes, you may be uncomfortable
. As a rule, if your supply is generally good, you don't need to obsess about feeding times, it would be a different story if you had supply issues. I would probably not advice skipping feeds before 6 weeks when your supply is well established but if you happen to forget once, don't worry.
Of course, if you can keep it regular and by the clock, it will be even better but in my opinion, anything in half an hour range is OK.
Good luck!