1) What is the “time possessive”?

English often uses possessive marking with time words to express duration/measure before a noun: two years’ experience, a day’s work, in three years’ time. It’s a compact alternative to of-phrases and to hyphenated measure adjectives.

• Possessive We need two years’ experience.
• of-phrase We need experience of two years.
• Hyphenated We need a two-year experience record.

With things we normally avoid possessive ’s (the end of the street), but time words are a major exception.

2) Spelling & form

3) Which pattern should I use?

• Possessive ’s concise and natural before a noun: three weeks’ notice.
• of-phrase more formal/literal: a journey of three hours.
• Hyphenated measure used when the time phrase modifies a noun: a three-hour journey. The time word stays singular.

Avoid mixing systems: *a three-hours’ journey (✗). Choose either three-hour or three hours’.

4) Equivalences & conversions

5) Very common time-possessives

• a day’s work · a night’s sleep · a week’s holiday
• yesterday’s news · today’s meeting · tomorrow’s weather
• in two years’ time · in a week’s time
• this year’s budget · last year’s sales · next year’s targets

6) When the possessive is not used

• Age & states two months old, nine months pregnant (✗ *two months’ old).
• Preposition of duration With for + period: for three years (not *for three years’).
• After nouns used alone Don’t add ’s to the hyphenated form: a four-hour delay (✗ *a four-hours’ delay).

7) The “of-test” heuristic

If of + time sounds natural, the possessive version is usually valid.

• Works experience of two years → two years’ experience.
• Works a drive of one hour → one hour’s drive.
• Doesn’t work *months of old → keep: two months old.

8) Punctuation & spacing

• Placement The apostrophe goes after plural -s: three years’; and before -s in singular: one year’s.
• Irregular plurals take ’s: two children’s bedtime.
• No double marking Don’t write both hyphen and plural-’s: two-hour vs two hours’.

9) Gallery of natural examples

• He lives one hour’s drive from the airport.
• Two years’ experience is essential for this role.
• The three hours’ journey through the valley was stunning.
• We’re planning a ten-day trip (= ten days’ trip).
• Submit the report in a week’s time, please.

In careful writing both two years’ experience and experience of two years are correct; the possessive form is more concise and idiomatic in modern English.