英语语法分析

'Ours' and 'their' are possessives.

ours和their是所有格代词。

《牛津词典》

Possessives are often used in expressions of time.

在时间的表述中，经常用所有格。

When you refer back to indefinite pronouns, you use plural pronouns or possessives, and a plural verb.

但指代不定代词的需用复数代词，物主代词及复数动词。