I just checked: On Linkedin it is indeed optional but they made it look like they force you (https://imgur.com/a/0wdI3Wy)

I almost always see the dates with a month.

CVs are primarily consumed by people who hire people, why wouldn't you listen to what they say?

If you had to be out of work for a year and a half for personal reasons, just put it there. Trying to conceal stuff can work, but might not; hence, isn't the most conservative idea. All the best.