In Australia, our main style manual gives different advice.

It says:

“It was previously conventional to use an apostrophe in expressions of time involving a plural reference, such as:

six weeks’ time
three months’ wages

“The apostrophe is now often left out. Again, the sense of the phrase tends to more descriptive than possessive.

“When the time reference is in the singular, however, the apostrophe should be retained to help mark the noun as singular:

a day’s journey
the year’s cycle.”

I think the traditional way that you recommend is better because it’s more consistent.

Michael Gladkoff

Inanimate possessives are very peculiar. Most style guides are clear that inanimate objects should not be possessive, with some exceptions (time and measurement). Some grammarians disagree: Motivated Grammar provides excellent full explanation here on this disagreement: http://motivatedgrammar.wordpress.com/2009/02/23/preposterous-apostrophes-viii-inanities-in-inanimate-possession/

I recommend that business writers do not use inanimate possessives. I agree with Motivated Grammar that it’s evolving, and the “rule” is adopted rather than prescriptive. However, it’s best to be extremely clear in business writing, so avoiding grammar controversy makes sense. I’d use an inanimate possessive in fiction and narrative if it rang better, but not in a business document.