in three years' time

[claig]

Is the apostrophe here just pedantic or is that how it should be written?

I think it's correct. I would think "in the time of three years" but it's not really a possessive so maybe I've been wrong for decades (not for the first time)

I had a discussion with my headteacher about this only last week. I've always used apostrophes in situations like your example, but she disagreed with me.

I'd love to know whether I'm right or not.

Yes I think so

One year's time
Three years' time

Is how I always write it

I always ask "who does it belong to?"

The car belongs to the woman. It is the woman's car.

The dog belongs to the boys. It is the boys' dog.

The time belongs to the three years so the apostrophe goes after the s.

Thanks everyone, I think the apostrophe should be used.