Quintus Curtius Rufus, History of Alexander 8.9.36

8.9.36

Lunae cursu notant tempora, non, ut plerique, cum orbem sidus inplevit, sed cum se curvare coepit in cornua, et idcirco breviores habent menses, quia spatium eorum ad hunc lunae modum dirigunt.

They regard as gods whatever objects they value, especially trees, to violate which is a capital offence. Their months they make to consist each of fifteen days, but they nevertheless assign to the year its full duration. They mark the divisions of time by the course of the moon, not like most nations when that planet shows a full face, but when she begins to appear horned, and hence, by fixing the duration of a month to correspond with this phase of the moon, they have their months one-half shorter than the months of other people. Many other things have been related of them, but to interrupt with them the progress of the narrative I consider quite out of place.