If the numbers on the back of the famous Fantasy manual are a date, then yes, it would be today! However, it’s only a coincidence because eight, 13 and 21 are part of the Fibonacci Sequence if you’re going from left to right.
To illustrate if you don’t care for the link: one, two, three (one + two), five (two + three), eight (three + five), 13 (five + eight) and 21 (eight + 13).
What those three numbers mean with the manual, the future of the show (I started over and I’m almost done with season four), Finn being a hero or how it will keep the Lich at bay, who knows.
SPOILER AHEAD
At this time, all I know about the Enchiridion is that the Bear who liked to imitate Finn gave it to a snail possessed by the Lich.
SPOILER OVER
Pretty cool how this will be the last Fibonacci string which will make a date on our calendar for the remainder of the 21st Century.