Can "zero hours contracts" be considered as not grammatical?

Zero hour contract (or zero-hour contract) can be found :

Number of workers on zero-hour contracts in UK rises to 801,000

Zero hour contracts are a hot topic in employment.

Zero hour contracts are history

But more frequent with an s:

Zero hours contracts: guidance for employers

Zero hours contracts have been in the news...

or hyphenated:

Number of people on zero-hours contracts in UK...

(to note that this Independent article mixes hyphenated and non hyphenated forms in the same article)

My question is not about zero+plural noun, I am aware of this ELU answer, but on the "s" being left when "zero-hour" is used in front of a noun, i.e. as an adjective.

I've noticed that "hours" is sometimes (but rarely though) used, in for instance, "eight-hours shifts" or "eight-hours day" but "eight-hour day" and "eight-hour shift" seem to be more frequent.

The only explanation I can think of is that an apostrophe ('s) has been lost somewhere in the process and thus should not correct spelling have been either zero-hours' contract or zero-hour contract? If not why are the usual forms "eight-hour day" (no s) and "zero-hours contract"?

Reference-work discussions of phrasal adjectives of duration or amount

In the larger context of how durations and amounts are usually handled in English when they take the form of compound modifiers, the oddball construction isn't "zero-hour contract," but "zero-hours contract."

Bryan Garner, Garner's Modern American Usage, second edition (2003) puts the normal rule for such compound modifiers as follows:

PHRASAL ADJECTIVES ... D. Duration or Amount. When phrasal adjectives denote durations or amounts, plurals should be dropped—e.g.: "The report doesn't disclose whether Annie Bell was born after a normal nine months pregnancy {read nine-month pregnancy}." Likewise, one should write 14-hour-a-day schedule, three-week hiatus, 32-year-old Kansan, 2,000-bottle wine cellar, and 25,000-volume library. The exception is with fractions, in which the plural is retained {a two-thirds vote}.

Other reference works implicitly or explicitly endorse the same rule. From The Chicago Manual of Style, fifteenth edition (2003):

5.92 Phrasal adjectives. A phrasal adjective (also called a compound modifier) is a phrase that functions as a unit to modify a noun. ... (5) If the phrasal adjective denotes an amount or a duration, plurals should be dropped. For instance, pregnancy lasts nine months but is a nine-month pregnancy, and a shop open twenty-four hours a day requires a twenty-four-hour-a-day schedule. The plural is retained only in fractions (a two-thirds majority).

From The Oxford Guide to Style (2002):

7.2 Units All units of measurement retain their singular form when they are compounded to form hyphenated adjectives before other nouns:

[Examples:] a one-way ticket; a two-mile walk; a three-minute egg; a four-litre engine; a five-pound note; a six-foot wall; a seven-league boot; an eight-hour journey; a nine-inch nail; a ten-gallon hat; a 100-metre race; a 1,000-megaton bomb

From Allan Siegal & William Connolly, The New York Times Manual of Style and Usage, revised edition (1999):

hyphen. ... Use the hyphen in constructions like three-mile hike and 30-car train and to avoid confusion in words like re-form (meaning form again).

From Margery Fee & Janice McAlpine, Guide to Canadian English Usage, second edition (2007):

hyphenation ... Hyphens are normally used in the following contexts: ... • in a phrase describing age length, etc. ('six-year-old child, '12-foot-high fence')

Although these references suggest unified support for using hyphenation plus a singular form of the unit of measure in situations calling for a phrasal modifier, actual usage is not so monolithic. Lady Jane Grey, for example is sometimes referred to as the Nine-Day Queen, sometimes as the Nine Days' Queen, and sometimes as the Nine Days Queen. And the prolonged European conflict of the period 1618–1648 is normally referred to either as the Thirty Years' War or the Thirty Years War.

One reference work—Words into Type, third edition (1974)—notes that inconsistency on this point sometimes arises in the context of common (non-proper) nouns:

Number and noun. Always hyphenate a compound in which one component is a cardinal number and the other a noun or adjective.

[Examples:] eleven-inch stick; ten-pound bag; a three-hundred-dollar clock; seven-pointed star ...

Strict observance of the rule is particularly important when the noun modified is in the plural, for without the hyphen the phrase might be misleading or at the least ambiguous. It is unsafe to depend upon the context to make the meaning unmistakable. Not the difference in meaning.

ten acre farms; ten-acre farms

two dollar tickets; two-dollar tickets

Usage is not strictly consistent in the number of the noun in the compound. Note, for instance: sixty-day note, six-months note, four-week period.

This last remark provides grounds for retaining the plural form of a plural noun in particular compound-adjective constructions for which (inexplicably) common usage goes against the grain of the vast majority of expressions of phrasal adjectives of duration or amount, as in "six-months note." That is the strongest case that I can muster for preferring "zero-hours contract[s]" (with or without the hyphen) to the expected conventional form "zero-hour contract[s]."

Google Books matches for 'zero[-]hours contract[s]' and 'zero[-]hour contract[s]'

A ramble through Google Books search results yields the following numbers of unique matches for the various competing forms:

zero hours contract: 32 matches

zero-hours contract: 14 matches

zero hour contract: 16 matches

zero-hour contract: 27 matches

zero hours contracts: 32 matches

zero-hours contracts: 14 matches

zero hour contracts: 18 matches

zero-hour contracts: 26 matches

Overall, the results for hour versus hours is quite close: 87 matches for "zero[-]hour contract[s]" versus 92 matches for "zero[-]hours contract[s]." Within the results, however, a strong predictor of whether the s is included in hour/hours appears to be whether the writer includes a hyphen after zero: When the hyphen is absent, the preference is heavily in favor of "hours": 64 matches for "zero hours" versus 34 matches for "zero hour." But when the hyphen is included the singular hour comes out far ahead: 53 matches for "zero-hour" versus 28 matches for "zero-hours."

So on the evidence from Google Books search results, I would say that whether you choose to keep the plural hours or make it singular hour is affected most strongly by whether you view "zero-hour contract[s]" as a noun (contract[s]) with a phrasal adjective modifier (zero-hour), in which case you would be prone to follow the general rule of adding a hyphen after the number and making the unit of measure singular, or whether you view "zero hours contract[s]" as a set phrase requiring no hyphenation, in which case you would be prone to leave out the hyphen and leave hours plural.

I did come two books in which the authors included an apostrophe after hours; but these instances are not among the earliest matches for the phrase. I found examples of "zero hours' contract" from 2001 and 2014, and of "zero hours' contracts" from the same two sources, but the earliest matches in the Google Books search results for "zero hours contract[s]" are from 1989, 1991, 1993, and 1994. The relevant text from the 1989 match (Ursula Huws, Jennifer Hurstfield & Riki Holtmaat, What Price Flexibility?: The Casualisation of Women's Employment doesn't show up in the snippet window, but in search results it appears as follows:

This situation has led to considerable legal inventiveness on the part of Dutch lawyers to ensure that workers' casual status is formalized. The 'on-call' contract has spread rapidly in recent years, most notoriously in the form of the 'zero hours' contract, which does not guarantee the worker any work (or income) whatsoever, but requires her to be permanently on-call in case of

It thus appears that "zero hours contract[s]" was not originally considered a possessive phrase. The earliest matches for "zero[-]hour contract[s]," meanwhile, are from 1991, 1993, and 1994.

Conclusions

The normal approach in English to situations in which a plural expression of duration becomes a phrasal adjective is to make the unit of measure singular—as in "24-hour party people"—but there are occasional exceptions to this tendency. Google Books search results indicate a very slight preference for retaining the plural in the phrase "zero[-]hour[s] contract[s]," meaning that that phrase is evidently one such exception.

The difference in total Google Search result matches between "hours" forms and "hour" forms of the phrase is very slight, however: 92 for the former versus 87 for the latter. A much wider split occurs when we focus on the presence or absence of hyphenation: "zero hours contract[s]" beats "zero hour contract[s]" 64–34, while "zero-hour contract[s]" beats "zero-hours contract[s]" 53–28.

The hypothesis that the phrase originated as "zero hours' contract[s]" finds virtually no support in the Google Books record, examined chronologically. Only two books that Google Books searches uncover use the possessive form "zero hours' contract[s]," and the earlier of the two appears 12 years after the earliest instance of "zero hours contract" overall. As between "hours" and "hour," however, the dating is much closer, with an earliest date of 1989 for "zero hours contracts" and an earliest date of 1991 for "zero hour contracts."

My guess is that "zero hour[s] contract" is a term of art (or professional jargon) that emerged from the overlapping fields of employment law and sociology, probably in the UK, as an abbreviated way to identify a type of contract that does not guarantee the contracting worker a minimum number of hours of paid work per day or week ("zero hours"). The earliest adopters of the term—being lawyers and sociologists—were evidently not especially concerned with whether it followed the general style rule for phrasal adjectives of duration or amount. Eventually, if dictionaries weigh in on the question of which spelling is most common, the language may standardize on a particular form; but for now we have nothing to go on but multiple individual preferences and closely divided usage.

It is common, but by no means universal, for a noun (or noun-phrase) used as a modifier to lose any plural ending. Counter examples are:

• solids model(l)ing
• pensions advice
• prices index

So the plural form is certainly not ungrammatical.

My suggestion for why some people say "zero-hours contract" is that the phrase "zero hour" exists, and means "the scheduled time that something is supposed to start", so they avoid the possible ambiguity by using the distinct form "zero-hours". (I am pretty sure that that is the motivation for "solids modelling", as opposed to "solid modelling", which might mean "a solid kind of modelling" rather than "modelling solids")

"Eight-hours" is not motivated in the same way, since "eight hour" is not an established phrase.