They only just added it, so you wouldn't have seen it before.
Really? It counted all the days I've been playing though, I swear I saw it before, hm
are they gonna add a button to check it?
Well there was already a counter implemented in the game to see how many days people have been playing for.
When a person hasn't played in for 10 days their rank bubble gets greyed out and you can't collect bonuses from their farm. I think they probably just used data from that.