I figured out when Janner's birthday is! Of course we know that it is in winter, and that spring came quickly after it, but I have a date! His birthday is...
March 14th!
In TWatWK, page 37, is says, "And so on the seventeenth day of Threemoon, the first winged Fangs descended upon the Hollows and the second battle of Ban Rona began." The seventeenth day of Threemoon is March 17th. I think it's safe to say the moons in Aerwiar line up with our months because of Dragon Day being on Andrew Peterson's birthday, and I think Andrew Peterson confirmed this somewhere. Anyway, March 17th began in Ban Rona on page 27 of TWatWK, with Leeli heading to the Guilding Hall with her dogs, which was "On the morning Janner set out from the forest."
That very morning, Janner woke just before sunrise somewhere in the Hollows and found a letter from Guildmaster Clout in his pack. In the letter, Clout said, "By the time you read this, you'll be feeling hungry. That's because you've been sleeping for two days." (TWatWK p. 20) Assuming the dosage in the muffins was as Clout expected, Janner had been sleeping all day on the 16th and all day on the 15th. This means that Janner fell asleep on the evening of March 14th - his birthday!
@ember wingfeather , I know we've discussed his birthday at length before, and I've finally found the evidence I needed! It looks like my first guess was correct!
And @J. M., a Throne Warden (that has ADHD and sometimes acts a bit more like a High King) to two sisters, a tinkerer/fixer of things, and a science, engineering, and aerospace aficionado extraordinaire , I'll try to get the timeline of TWatWK written out soon! I'm going through the book again right now!
https://wingfeathersaga.fandom.com/wiki/Janner_Wingfeather