所有的票在两个月内有效。

All tickets are valid for two months.

《柯林斯英汉双解大词典》

这项优惠仅在限期内有效。

This offer is available for a limited period only.

《牛津词典》

这张票三个月内有效。

The ticket is good for three months.

《牛津词典》

Now, how in the world can somebody actually make that work for a whole country?

全世界有谁能使这种体系,在全国范围内有效实施？

耶鲁公开课 - 金融市场课程节选

But it's only an effective vaccine in the context where you have people who can go to the doctor's office, or to an urgent care, some kind of medical facility, three times, reliably, at a specified period of time.

但为了保证接种效果,人们必须亲自去诊疗室,或者在急诊室里,使用一些医学设施辅助接种,还需要接种三次,它只在一段时间内有效

耶鲁公开课 - 生物医学工程探索课程节选