A calendar system for Mars has already bean devised for potential future settlement. The calendar contains 24 months of 27 or 28 days each, named after Latin and Sanskrit constellation names.
France had its own calendar for only 12 years starting in 1792. Each week had 10 days, each day 10 hours, each hour 100 minutes, and each minute 100 seconds.
The Kodak company used a 13-month calendar from 1924 to 1989 and George Eastman pushed for its worldwide acceptance.
In North Korea it is the year 103, not 2014. This is because their calendar is based around the birth of Kim Il-Sung.
The days between 4 October 1582 and 15 October 1582 don’t exist in history.
Our calendar repeats itself every 400 years.