Explore and prove your dominance over all 24 floors of the Proving Grounds, with the floor corresponding to today's date.
Refers to in-game date.
When in PG, if the calendar date in game (visible with 'time') matches the level you are on, you have a high chance of this triggering. Roughly 1-10 kills should be sufficient. A message is displayed that you have gotten that floor in the kill spam. Additionally you can see in your personal log what you have finished. The date is the highest chance, but it is somehow possible to get them for other floors as well. I got 10 on Feb 3rd in game.
Log output:
5 18 19 23 1 2 10