The Imperial Calendar is divided into 10 months of 40 days each, and is approximately twice as long as a Terran year. The effect of this is that dynasty members age by one year on their birth's month, and another year after 5 months.
The months are:
Each game day is two hours long in real time, making one game month roughly half a week long and one game year roughly one month long.
It may be be of interest to know that days change at even hour and at the 20 minutes mark.
The date on the calendar is marked based on the ruling Emperor at that time. this causes some oddities with trying to convert reign dates to a coherent timeline, as the last year of the previous Emperor will also be Year 1 of the new Emperor. When calculating a timeline this way, one has to be careful of that overlap. If multiple Emperors are deposed within the same game year, this causes even more confusion.
All of the galaxies have their own coherent calendars. Baade reckons the years in Imperial E… (IE);Capricorn calculates years After Founding (AF, starting at 300); Draco Reborn was now counts the years After Reunification (AR); Eridanus counts the years of the Coriolanus Era or Common Era(CE); and Gemini, due to lack of other dating, seems to have a few dates noted in the Capricornian fashion.