The Calendar of Tos is used by convention wherever humans congregate. Other races use the human calendar when dealing with humans but use their own methods of reckoning time when referring to internal affairs. The calendar is lunar. Months are 30 days each.
Years are numbered from some (presumably) important date. The current year at the start of the campaign is 1313.
The moon is a greenish-white, cloudy disk that waxes and wanes like, though conveniently more regularly than, our Moon.