As Hasrat has stated, this is actually intentional - players back in the Beta test mentioned this and SE responded it's working as intended, which means as bizarre as the Eorzean calendar seems to be, it's how it's meant to be. The fact is if you're hoping to have a 1-1 exact match for Eorzean day with a specific real-world Gregorian calendar day (such as your real life birthday) it's not really possible. So definitely working as intended I'm afraid.