The Annus Novus Decimal Calendar System (AND) is a fictional calendar for the alternate world Empire of Atlantium where it replaces an earlier attempt to reform the Gregorian calendar, which had a 400-day year divided into 10 months. AND is also known as IDS for Imperial Decimal Standard. It seems to have been developed around 1990.
The AND calendar has 10 months of alternating 36 and 37 days each for a normal year of 365 days and 5-day weeks that are not fully aligned with the months. It reuses the Gregorian leap rule and has the leap day at the end of the year in a separate month called Intercalarius; it also doesn’T belong to any week so the calendar remains perpetual. Its epoch lies 8519 years before that of the Julian/Gregorian calendar, so there‘s an offset of +281 or −119 to the start of the Gregorian era.