One of the things that always causes confusion and that is hard to find in the documentation is making date calculations in Org. I was reminded of that when I saw a reddit question on making date calculations in an Org table. The comments to the question give some useful hints.
After I read the reddit query, I remembered that Sacha had, of course, already already covered this material so you should take a look at her post too. One of the things that Sacha points out is that it’s important to use “proper” Org dates—that is, active or inactive dates with the […] or <…> brackets. That ensures that you can perform arithmetic on them directly without having to worry about converting from string to date.
As I said, these two posts are useful because it’s hard to locate the documentation that covers the subject. Once you get in the habit of storing information like this in Org tables, you’ll find that techniques like these are really useful.