My family are so bad for this and often it's just to "pop in" so I'm not even cooking or anything but if someone tells me they're coming over at 2 pm I expect them at 2. I make sure we're ready and the house is clean, I don't do anything that might keep me from answering the door like cleaning or vacuuming upstairs in case I don't hear the door... so I'm basically just waiting around and it annoys me so much. I'm then in a bad mood when they get here.
I'm sure I shouldn't be so bothered but I can't help it if I'm expecting someone at a certain time.