Warning: Numerology (the most insidious form of mathsturbation).
In the fall of my freshman year at college, I reread The Illuminatus! Trilogy, keeping a log of every character and the first page they were introduced in. I also examined the calendar they used, and decided to make some modifications.
The Christian (Gregorian) Calendar we use today is seriously flawed. The length of each month varies greatly from 28 days to 31, Leap Days are inserted into the calendar at the end of the shortest month according to three rules based on the number four, and yet the calendar is only accurate for 3300 years.
My Illuminated Calendar is based entirely around the number five. The calendar year is broken down into five seasons of 73 days each, corresponding to the five stages of any time-based system with the classic example being the theatrical form of Introduction, Rising Action, Climax, Falling Action, and Conclusion, which is exemplified by the five-act plays of Shakespeare, the five acts of the hour-long drama, and the five seasons of Babylon 5.
Each season is then broken down into five months, the tricycle of three 15-day months, and the bicycle of two 14-day months. Each of these months is named after a character introduced in the corresponding part of The Illuminatus! Trilogy:
Great effort was taken to avoid naming months after characters that have an important and surprising impact on the novel, as well as avoiding characters with offensive names. Great effort was also taken to introduce the amusing conflicts with the Gregorian Calendar represented by the months of August, A, Friday, and One.
Effort was put into the Leap System as well. Whereas the Christian Calendar isn't even accurate for 4000 years, my calendar is accurate for over a million, by introducing five simple rules centered around the number five, and each rule affecting a different seasion according to its frequency: Adding a day every 5, 25, 625, and 500 000 years, and subtracting a day every 100 000 (I think. I misplaced this rule and the 14-digit number of days per year). Each rule is applied to the season corresponding to its frequency, adding a day between the tricycle and bicycle. Humorously, that means a day is subratced from the season called "Bureaucracy," resulting in a bicycle of 15-day months followed by a bicycle of 14-day months.
For convenience sake, I had the first day of the year six thousand AM match 2000 AD, resulting in a one-day difference most of the time. I have kept it up since then, and hope to continue to do so for the rest of my life.
You may wonder just what posesses me to do such a thing. I dislike the fact the people assume that since the Christian Calendar is widely used and "good enough" for thousands of years of use, that they forget that it is a calendar, not the calendar. Indeed, several other calendars are still in wide use, most notably the Chinese and Hebrew calendars. I insist on using AD whenever I formally record the date in the Christian Calendar, to indicate clearly which calendar I use. And people DO notice this. And they do appreciate the fact that I recognize that the Christian way of recording time is not the only, and most definitely is it not the best.