My instinct is those are relatively recent, say in the last fifty years or so - but it's worth pointing out that no speaker of American English would be confuse

My instinct is those are relatively recent, say in the last fifty years or so - but it's worth pointing out that no speaker of American English would be confused by "a couple of weeks ago". A counter example could be the way British English elides the definite article, like "when I was in university" or "she was in hospital for a month", which sounds strangely clipped to speakers of American English.