Mmmmmaybe, but Autumnia just found another month name (from SC), in which a Narnian Owl places the start of Rilian's troubles in May. So my suspicion is that either the Narnian calendar uses thirteen strictly 28-day months with Greenroof being the interpolation into 'our' calendar, or the Telmarines brought this-world names which have gradually overwritten the old Narnian names, and the Galman (Galmian?) sailor is using those so the rest of the crew will understand him.

(Or, of course, Greenroof could simply be a poetic name for July or August. Perhaps all the months have doubled names!)