Reading the numbers
X shows a join month under every bio, but not the day. The profile data behind it carries the full timestamp, so the tool can give the day and count the days since. The post rate is the account's lifetime post count divided by its age, which is a rough average: an account that posted heavily for a year and then stopped shows the same rate as a steady one. For the current rhythm, run the profile analyzer, which reads the last 60 posts.
Accounts created since 2016 also carry their creation time inside the numeric user id, the way post ids do; the tweet ID to date converter can read it without a lookup. To weigh age with the other signs of an automated account, see the Twitter bot checker guide, and to check a whole follower list at once, the fake followers checker.