Apostrophes

[Uncle M]

Does anyone have a view on the use of apostrophes in such expressions as:
12 months guarantee and 6 months experience?
I feel that the word months' should be written thus, with the apostrophe after the s.
However, it's not a possessive since the the guarantee does not 'belong' to the number of months. Nor are the words '12 months' a compound adjective; otherwise one would write '12 month guarantee'.
On the basis that there are missing characters (6 months worth of experience), I prefer to use the apostrophe in text, but do not object to removing it for aesthetic reasons in headlines.
Guidance on the use of apostrophes is relatively easy to find, but rarely have I seen a definitive answer to this example.
Anyone?

Dave

[trans]

I wonder either, just only that the compound adjective would write with a "-" thing (12-month guarantee), wouldn't it?

[Uncle M]

A hyphen, yes - you're right.
One of the problems the graphics industry faces is when to sacrifice precision of grammar for the sake of attractive display style.
Are there any other copywriters, sub-editors and proofreaders out there who face the same dilemma?

[Tdol]

The problem with removing it in headlines is that this is one of those issues that those of a pedantic bent feel strongly about- missing apostrophes seem to wind them up.