There's an inconsistency with the date chosen for a birthdate and which date actually gets implemented in your character's biography.

Essentially, one of the dates repeats itself over two days, throwing off the numbering. To see this, pick a month and then hover over each date until you notice that one date right after the other doesn't change the actual date underneath the date sentence (the [??/??] number beneath the "Xth Sun of the Xth Whatever Moon" phrase).

Incidentally, every months fall into one of the four categories below on what the numbering error is:

Months where 28th and 29th are both listed as 28th:
First Astral, Second Astral, Fourth Astral, Fourth Umbral, Fifth Umbral, Sixth Umbral

Months where 7th and 8th are both listed as 7th:
First Umbral, Second Umbral

Months where 29th and 30th are both listed as 29th:
Third Astral

Months where 7th and 8th are both listed as 7th and 29th and 30th are both listed as 28th:
Third Umbral, Fifth Astral, Sixth Astral