310 - track 1 (medieval song)
421 - track 2 (not sure what this is)
118 - track 3 (fucking metroidvania)
333 - track 4 (pippa the ripper)
401 - track 5 (uruka's theme)
So it's birthdays if we look at Pippa and Uruka's numbers. So their birthdays are March 10th, April 21st, and January 18th. I just entered in 333 because I know that's a meme for Pippa and then I entered in Remi's and Panko's birthdays since they were mentioned in the book but got no hit. Uruka's birthday did have a code though, so I just went through and started powering through with month and dates.