What if he's adding a new day to the calendar (6/31)? Or maybe it's a date that is 6 years, 31 days from when we saw it first? Maybe it's a clue for the Mistborn Movie? What if there are 631 members of the Ghostbloods or one of the many spren groups?
The 631st WoB on Arcanum is this one from the Well of Ascension annotations.
631 years ago was the year 1393.
Splitting 631 into 6/31, the sixth event and 31st wob in that event, we get this one from the Final Empire Annotations.
I could do it the other way, as in 63/1. That wob is from the Alloy of Law release party:
Starting from the latest WoBs on Arcanum, the 6th event /31st wob in event doesn't exist. Brandon's book club- sunlit man only goes up to 13 wobs.
631st starting from most recent gives us:
Most likely, none of these are useful, but they might be.
Edit: I just realized that 6+3+1=10, the Vorin holy number, the number of Roshar, the number of the Bondsmiths- the tenth order.