1 Iki Piki: Phew! How much real time do we have until the ship crashes?
1 Spanners: 30 seconds.
2 Iki Piki: How much subjective time do we have here in cyberspace?
2 Spanners: A fortnight.
3 Iki Piki: Is that long enough to install the software update, reboot, and then troubleshoot and resolve all the missing dependencies?
4 Spanners: It'll be tight.

I am continuously boggled by the fact that American English considers the word "fortnight" to be quaint and archaic, when it's a perfectly normal and frequently used word in other brands of English. The other day I ran across a dictionary entry for "fortnight" somewhere on the web, and it said that the word was "British slang for a two-week period" (emphasis mine).

Slang.

Slang.

Honestly, this is just like saying "baseball" is "slang for a type of bat and ball game", or "apple pie" is "slang for a pie crust filled with stewed apples". Or indeed "week" is "slang for a seven-day period".

"Fortnight" comes from Old English, and dates to before the year 1000. Shakespeare uses it multiple times in his plays. A search of current UK legislation finds 47 instances of the word "fortnight" enshrined in UK law. A similar search via the Australian Government's Federal Register of Legislation finds 2204 instances of the word "fortnight" in Australian legislation^[1].

"Slang" indeed.

[1] There doesn't seem to be a linkable direct URL to the search results, but go here, type in "fortnight" in the top search box, select "full text" under the "Choose contents" drop-down, and hit "Search".