To understand the New Britannia Calendar you first must need to understand that the time scale between Earth time and New Britannia time is precisely 1:24 and that New Britannia time (like Earth time) is persistent for online play.
1 hour of Earth time (or real time) equals 1 day in New Britannia.
Every New Britannia month is 28 days with 12 months to the year, or 336 days.
This also means that a year in New Britannia is exactly a fortnight on Earth.
This is useful to understand because it makes time in New Britannia predictable. The New Britannia new year for example always falls on the same hour of the same day every other week. Naturally the same holds true for any date on the New Britannia calendar. This simple fact remains true through each game update.