19:00 for 19:30

I am seeing an unusual (for me) phrase on invitations. Instead of just giving a straight out start time for a party or a lecture, an invitation will instead ask you to arrive at a "time for a time", for instance 19:00 for 19:30. I thought I knew what it meant but I decided to look it up just to be on the safe side. I found it is a British invention.

According to British social custom, "An invitation might state "7:30 for 8:00" in which case you should arrive after 7:30, but before 8:00 p.m., and preferably no later than 7:50 p.m." Another puts it more plainly, "you should not arrive too promptly for social events - but aim to arrive a respectable fifteen minutes after the specified time; thus, if a dinner invitation states '7:30 p.m. for 8:00 p.m.', it means that you will be expected at about 7:50 p.m."

I like it but it definitely takes the fashion out of being fashionably late.