Help. Apostrophe issue Whose right and if so, why?

[Minnieisthedevilmouse]

Can someone please help explain something thats driving me bats? Ive been corrected at work for this sentence...

Accordingly the sentence, your sentence on each of the counts to which you pleaded guilty is one of five years imprisonment.

The correction states: five years' imprisonment

I thought an apostrophe here denoted possession or a missing letter? There is no missing letter. Unlike "it's". And I'm unsure why five years "owns" the imprisonment. Is this a language thing or a Law thing?

Help! Sincerely frustrated,
Minnie
:(

It's a reworking of imprisonment of five years. Like the hat of the boy, instead of the boy's hat.

That's how I look at it.

(And while we're on apostrophes - your title should be who's not whose.)

It requires an apostrophe because it means "imprisonment of five years".

Also... something that's driving me bats ... I've been corrected ... (comma after "guilty").

It's like the film Two Weeks' Notice. The way Lynne Truss explains it is that we would quite happily write "one week's notice" but we get all confused as soon as it becomes more than one.

So you would have written "one year's imprisonment" (I assume) but it's the more than one that's thrown you.

And yes, it is "who's right" and "that's been driving me bats" as well...

...and a comma after "guilty"...

and "I've been corrected at work"...

This is a great website.

Yeah I know about the others. I'm not at work now. And I've had wine.

Thanks all. Much appreciated. I guessed I was wrong and obvs said nothing but half smiled graciously. But hated the supercilious correction without explanation. That's just rude. (Jumped up twerp.)

Thanks for link too.