'Ours' and 'their' are possessives.

ours和their是所有格代词。

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。

The purdue online writing lab also explains how to use an apostrophe correctly in possessives and other cases.

普渡在线写作研究室也解释了如何在所有格及其它时态中如何正确使用撇号。

While we can talk about John's car or Mary's car, or John and Mary's car if it belongs to the two of them, we are in difficulty if one of the possessives is a pronoun: John and her car.

虽然偶们可以的对约翰哒汽车或玛丽哒汽车，或约翰和玛丽哒汽车谈，假如它属于他们两个，偶们是的在困难哒代名词：约翰和她哒车假如所有格之一。