Once you poke at it, the common notion that Rosh Hashanah marks the creation of the world is not so simple. Maybe Tishrei isn't even the first month of the year, but the seventh!
On Musaf of Yom Kippur, we recite piyyutim that recall the special avodah service performed by the Kohen Gadol. Might they be based upon a textual model written by an actual Kohen Gadol or other eyewitness to the avodah in the Beit ha-Mikdash?