Excel Question

I am creating a spread sheet tracking qualifications. these quals have an expiration date of anywhere from 2 years to 3 years. Here is what I am looking at. Roughly 15 people and roughly 20 quals with different expiration dates. If I highlight each column (tasks) I think I do a conditional format but how do I designate 24 months or 36? The dates here are when they got the task, I want to conditional format it to change colors when a person is a month away from it expiring. I did this once before but can't remember how I did it