How to use TODAY() to conditionally format due dates?

I am a complete noob at excel so my apologies if this is very easy.

Essentially, I am trying to make a column where I write the last date I received records. I’m supposed to get updated records at least once every 90 days, but frequently they’re late. I want the cell to turn red if the date in it is older than 90 days. The column is K and the first cell with info is K2.

I tried =K2<TODAY()-90 and =$K$2<TODAY()-90, but the test dates I tried either don’t turn red when they should or *all* of them turn red even if left blank.

What formula should I use?