语法的问题,是单数时候就用year's,s 在' 的前面：one year's time,一年的时间.
当你说很多年的时候,复数years,因为语法上不想让你写成years's,所以就写成years' time:4 years' time,4年的时间.
举例：
Apostrophes are used in time expressions (e.g.,three years' experience,two days' pay,one day's time).These are also known as temporal expressions.
In a temporal expression,the apostrophe is positioned before the s for single units of time and after for multiple units of time.For example:
I never did a day's work in my life.It was all fun.(Thomas A.Edison)
(As this is one day,the apostrophe goes before the s.)
Alan was given two days' notice.
(As this is two days,the apostrophe goes after the s.)
That is the equivalent of one year's pay.
(As this is one year,the apostrophe goes before the s.)