I found last night that Hesychius of Alexandria in his Lexicon of unusual and obscure Greek words written in the 5th or 6th c. CE includes the following entry:
δεκάτη προτέρα· ἡ πρὸ εἰκάδος, ὡς ὑστέρα· ἡ μετ' εἰκάδα
δεκάτη προτέρα· ἡ πρὸ εἰκάδος ("for/instead of εἰκάδος/20th"), like ὑστέρα· ἡ μετ' εἰκάδα ("after εἰκάδα/20th, i.e., 21st")
PS: Note - I'm getting that use of πρὸ from LSJ, definition III.1.
Henry George Liddell, Robert Scott, A Greek-English Lexicon, πρό
III. in other relations:
1. of Preference, before, rather than, κέρδος αἰνῆσαι π. δίκας to praise sleight before right, Pi.P.4.140, cf. Pl.R.361e; πᾶν δὴ βουλόμενοι σφίσι εἶναι π. τῆς παρεούσης λύπης anything before, rather than, their actual trouble, Hdt.7.152 (so, in order to avoid, “π. τοῦ δεινοτάτου” D.54.19); “πᾶν π. τοῦ δουλεῦσαι ἐπεξελθεῖν” Th.5.100, cf.4.59; ἑλέσθαι, αἱρεῖσθαι, or κρῖναί τι π. τινός to choose one before another, Id.5.36, Pl.R.366b, Phlb.57e; π. πολλοῦ ποιήσασθαι to esteem above much, i.e. very highly, Isoc.5.138; “π. πολλῶν χρημάτων τιμήσασθαί τι” Th.1.33, cf.6.10; π. ἄλλων more than others, Pl.Mx.249e (v.l.), cf. A. Th.1002; δυσδαίμων . . π. πασᾶν γυναικῶν ib.927 (codd., lyr.); “π. πάντων θεῶν τῇ Ἑστίᾳ πρώτῃ προθύειν” Pl.Cra. 401d: after a Comp. it is redundant, “ἡ τυραννὶς π. ἐλευθερίης ἀσπαστότερον” Hdt.1.62, cf.6.12, Pl.Ap.28d, Cri.54b, Phd.99a; for ἤ after “ἄλλος, οὐδεὶς ἄλλος π. σεῦ” Hdt.3.85, cf.7.3.
That's an ancient author providing specific definitions for both δεκάτη προτέρα and δεκάτη ὑστέρα as alternative names for the 20th and 21st days of the month.