Fortnightly, biweekly etc etc

[Spot's Meow]

The term fortnight is quite familiar to me because I enjoy reading 19th century American literature, where it is used often. However, now that I think about it, I don't hear the people around me using it, so I suppose it's not a common word anymore. When describing these lengths of times myself, I avoid the ambiguous terms altogether and just use "twice a week," "every two weeks," "twice a month," "every other month," "twice a year" (or sometimes "every six months"), "every other year," etc.

 

[RJDiogenes]

Another time related one that confused me reading Stephen King the other day - 'Quarter of five' - is that quarter to, or quarter past?

Quarter of 5 would be 4:45, while quarter past is 5:15. This is very common usage here, so it's not surprising that it would be in a King novel.

 

[MacLeod]

age in the UK would normally be x to y, or x past y. i.e. Quarter past, though fairly often it'll be said as "five fifteen". Though when it comes to writen time the 24hr clock is more common. i.e Bus/Train timetables are all in 24hr clock. the BBC One website, gives programme start times in 24hr clock.

 

[cultcross]

but I've never heard of biannual being used to refer to every 2 years in any official context. Surely it's just simple incorrect usage?

The word has both definitions according to the OED (I'd link, but it's behind a paywall). American Heritage also includes both.
Wiktionary gives the definition of 'every 2 years' as 'proscribed' but gives a few quotations from publications using it in this form.
I don't know about official style guides, but I remember when being taught to write for scientific journals in my former life, we were told to avoid the word because of the ambiguity.

 

[Lindley]

I've never heard of biannual being used to refer to every 2 years in any official context.

The US Federal Aviation Regulations require every pilot to have a flight review "within the last 24 calendar months." Although the word is not present in 14 CFR, this is colloquially referred to as a BFR, or Biannual Flight Review.

 

[Holdfast]

but I've never heard of biannual being used to refer to every 2 years in any official context. Surely it's just simple incorrect usage?

The word has both definitions according to the OED (I'd link, but it's behind a paywall). American Heritage also includes both.
Wiktionary gives the definition of 'every 2 years' as 'proscribed' but gives a few quotations from publications using it in this form.

Interesting. I certainly wouldn't ever use it in that context.

I don't know about official style guides, but I remember when being taught to write for scientific journals in my former life, we were told to avoid the word because of the ambiguity.

Yeah, avoiding seems the best solution. Funnily enough, I'm just going through the author review process process on a book I've written, and one consistent theme of the editors' changes is simplification and avoidance of ambiguity wherever possible! It's amazing how much ambiguity there is, if you really, really think about a sentence.

I just tried to look up usage around this area in Strunk & White, but can't find my copy. Not sure if it would be in there anyway, and it's a bit too American for me as well. The Oxford University Style Guide doesn't mention it as far as I can see.

The BBC support my definitions, and your suggestion to avoid ambiguity.