x,y are points in time for users A and B respectively.

OK, so you want to find groups of users who have posted at times near each other. Presumably looking for patterns like this: https://i.imgur.com/nK7i6Pv.png

As I write this, it sounds like a lot of work.

You could make a matrix where each row is an author, each column is a date, and the values could be 0,1, or a fraction of the user's total posts.

The problem would be doing pearsonr on each combination. We're talking 85,372 unique accounts for 2018. That's not something I can really do. Even if I could, I'm not sure how you'd group more than 2 users together, or what the desired output format would be