Seems like they're preparing a new part of Time Temple.

It's the door to the past. Past of the Verdure, Frozen Past, Burning Past, Forgotten Twilight...do any of these map names ring a bell? How about their connotative and denotative relation with the past? xD

I was reading this and wondering why no one had posted this...
The destroyed temple is the present (lolwut). It exists in the past, and the destroyed monks and guardians we see in the demon slayer's story are the ones that we've been killing in the past.

Also, there's a reason why the middle door would be the door to the present: it's in order.