Recognition that the ancient Greek phrase τῃ προτέρᾳ δεκατῃ (“the early tenth”) refers to “the twentieth” seems to have been acknowledged by Stephen White, the most recent translator of Diogenes Laërtius with whom I am familiar (2021):
“'Out of the revenues we have given Amynomachus and Timocrates, they are to set aside portions, in consultation with Hermarchus and so far as possible, for the sacrificial offerings for my father, my mother, and my brothers, and for conducting the customary birthday feast for us every year on the twentieth of gamelion, and likewise for the gathering of our fellow philosophers held on the twentieth of every month in memory of us and Metrodorus. they are also to join in celebrating the feast day for my brothers in Posideon; and they are to join in celebrating the feast day for Polyaenus in Metageitnion, just as we have done.'” (Lives of Eminent Philosophers translated by Stephen White)
Nearly everyone else from Gassendi to Mensch seem to overlook this nuance.