As far as I know, the rank upgrade is not related only to time and number of posts. It also depends on the quality of the posts, which makes the math.. not that obvious.
But I don't actually know how it's done now, but I guess that it involves some human check.
What ? That's impossible if you ask me , how they will check thousands & thousands of users lol besides I don't remember having a dely on time for my upgrade
I never heard of that my self , I'm pretty sure that's all it's all about math here , just reach the Activity needed and boom , here you are on the new Rank . nothing else
I hope someone can confirm tho