Let's say we have an user Bob. User Bob logs in into our *web* application every so often. We are a website that tries to save as little information about our users, however we'd like to know which users are inactive so we can suspend their account.

Is there any way we could save information about Bobs logins in a way that doesn't tell us anything about when he logged in, but tells us how many times Bob logged in in last `n` days?

Assume everything is being done on server side, and only thing we're trying to prevent is data leak in case of a database breach or similar.