Time and notice; which is possessive?

[bowdery2005]

Hi, a problem was posed to me yesterday and I simply cannot put it to rest!
The phrase "Seven days notice" has me baffled as to its punctuation. At first I thought it correct to follow the basic rule of possessive time, which leaves "days'" as the possessive noun. However I can also see that "day" is not a measure of time; and could be construed as a quantitative descriptor of notice.
If anyone can help me validate either argument I would very much appreciate your input.
Many thanks,
John

[MrPedantic]

Hello John

It's more usual to put the apostrophe after days, in official documents, etc., e.g.

1. ...ten days' notice...

= "notice of ten days [in duration]".

Some might argue that we can treat "ten days" as an attributive noun phrase, and omit the apostrophe:

2. ...ten days notice...

But when we use phrases of time attributively, we're more likely to say e.g.

3. ...a ten-year sentence...

than

4. ...a ten years sentence...

MrP