Since Araevis follows a similar orbit around the sun (Caesar), the year is a similar length: 365 days.
Since the moon (Aestus) has a similar orbit around Araevis, the length of a week is the same: 7 days.
Monday Tuesday Wednesday
Since the weeks are of a similar length, so too will months be. 4 weeks (or roughly one full lunar cycle). The 'New Year' will be in March, in the beginning of Spring, rather than in December, on the winter solstice.
The starting year of the RP is 500 P.T. or 500 post terminus. They begin counting following the Calamity.
Years prior to the Calamity are referred to in one of two formats. The Calamity occurred on Umbra 1st, 3238 A.V. (3238 anni viskar) or Umbra 1st, 1 P.C. (1 prius cladem).
AV is the era that people counted years by prior to the Calamity. It lasted for 3238 years. The years are counted positively.
PC counts negative, similar to BC or BCE. 1 year before the disaster, 2 years before the disaster, and so forth.
Both are considered acceptable by current scholars, and which one is used is largely dependent on context.