Page 448 - Practical English Usage 3ed - Michael Swan, Oxford

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However, both structures are possible in some expressions.
theearth'sgravity OR thegravityoftheearth
the plan's importance OR the importance o f the plan theconcerto'sfinalmovementOR thefinalmovementoftheconcerto the train's arrival OR the arrival o f the train
the world's oldest mountains OR the oldest mountains in the world
(NOT••• tt!fhe W8rltl) - see 139.7
Unfortunately, it is not possible to give useful general rules in this area: the choice of structure often depends on the particular expression.
subject
Note that the's structure often corresponds to a sentence in which the first noun is the subject of have or some other verb.
Joe's brother (Joe has a brother) the dog's tail (the dog has a tail)
America's gold reserves (America has gold reserves)
the manager's decision (the manager made a decision) Harris's novel (Harris wrote a novel)
In a few cases, the first noun may correspond to the object of a verb. the prisoner's release (they released the prisoner)
measurement of time: a day's journey
The's structure (or the plural with s1 is often used to say how long things last. a day's journey twenty minutes' delay
Noun + noun structures are also possible in expressions with numbers (see 386.5).
a three-hour journey a twenty-minute delay other expressions of time: yesterday's news
We can also use the's structure to talk about particular moments and events.
yesterday's news last Sunday's match tomorrow's weather worth
Note the use of the's structure before worth.
a pound's worth ofwalnuts three dollars' worth ofpopcorn
For the's structure in compound nouns (e.g. a doll's Ilouse, cow's milk), see 386.
possessives (3): my, your etc What kind of words are they?
My, your, his, her, its, our and their are determiners (see 154), and are used at the beginning of noun phrases.
my younger brother your phone number their plans
They are also pronouns, because they stand for possessive noun phrases: my younger brother means 'the speaker's younger brother'; their plans means, for example, 'those people's plans' or 'the children's plans'.
They are not adjectives (although they are sometimes called 'possessive adjectives' in older grammars and dictionaries).
possessives (3): my, your etc 441
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