Possessive apostrophe

[sazzle]

Which of these is correct? I am having a discussion with our copywriter at work over the apostrophe:
"After 2 days riding, the only thing that hurt was giving back the keys"
"After 2 days' riding, the only thing that hurt was giving back the keys"
In an ideal world we could rephrase the sentence but as this is a direct quote we can't change it. Can anyone definitively state which version is correct and also explain why one is right and the other is not?
Thanks

[mykwyner]

This is called "the genitive of measure."

"...2 days' riding..."

"...a dollar's worth of peanuts"
"...an hour's delay"
"...three hours' delay"

[MrPedantic]

Hello Sazzle, welcome to Using English!

I would use an apostrophe:

1. "After 2 days' riding, the only thing that hurt was giving back the keys."

This is known as the "temporal genitive". It denotes "during", e.g.

2. This week's news = the news during this week.
3. Last night's party = the party during last night.

Your sentence is a little deceptive, because the genitive plural of "days" is "days'" and therefore sounds the same as the ordinary plural. Also, the gerund "riding" looks like a present participle. And "two days" looks like the prepositional object of "after", whereas "riding" is the true object.

But you can neutralise these deceptive aspects by changing "two" to "a" and "riding" to "holiday":

4. After a day's holiday, the only thing that hurt was going back to work.

You wouldn't say "after a day holiday", which is the equivalent of "after two days riding". Thus the apostrophe is required.

All the best,

MrP

[MrPedantic]

Sorry to tailgate you, mykwyner -- you weren't there when I started.

MrP

[mykwyner]

No problem, I love reading your precise expositions.

[sazzle]

That's cleared it up - many thanks.