The Calendar
The calendar system used in Arthea today was established in 823 by William Corgart, whose northern kingdom used it to regulate both taxation and trade. Corgart's kingdom was so successful at trading that this calendar was soon used throughout the free lands. Nobody had a choice here - Corgart's nation was rich and had become powerful through trade. If nations wanted to trade with Corgart's realm (and most did) they had to use the Corgart Calendar.
It is based on a calendar that pre-dated The Great Disaster, and which was still in use among elves. The human and dwarfish societies had all but abandoned their original calendars when their society broke down. The use of twelve months had largely continued, as this is based on the cycle of the moon, but there were regional names for them and different year-numbering or naming systems, and many different days for the 'start' of the year.
Gnomes and dwarves had begun using a system that allowed each year to be named after types of precious or semi-precious stones in a repeating cycle which lasted eight years (there was the year of the 'ruby', 'aquamarine', 'diamond', 'opal', 'anthendorite', 'emerald', 'sapphire' and 'quartz', in that order). This system had worked well, but did not support long-term trade agreements reliably.
For Corgart, the calendar was a way to both appease his own powerful traders, and a way to leverage his kingdoms powerful trading position. It also gave him a taxation framework which made him personally very wealthy.