then why is 'a fortnight' not accepted, since the reverse would have to be accepted ?
Just to be impish I answered with "a fortnight" on 3/16/18, totally expecting to get flagged and was genuinely ASTONISHED to see it get accepted! (I'm from the states). So it 's all good now, mate.
Have you reported it. Do they say a fortnight in America, as that seems to be used it preference to English English.