This is a lunisolar calendar that I created. It is my attempt at creating a calendar that stays in synch both with the phases of the moon and, on average, with the solar year over a long period of time.
Convert from Gregorian date (only works for dates past 1939/3/20):
Year Month Day
Convert to Gregorian date (only works for positive years):
Year Month Day
My first aim was to have the months stay close to the phases of the moon, with each month starting near the new moon. But since there isn't a nice, even number of lunar cycles in a year, some years end up with twelve months, and others end up with thirteen. This is pretty typical of lunisolar calendars. Mine follows an eleven-year cycle that goes like this:
Months alternate between 29 and 30 days in length. The general rule is that odd numbered months have an odd number of days (29), and even numbered months have an even number of days (30). However, when a year has a thirteenth month, it is always a 30-day month. Additionally, every five years is a leap year. When the year is divisible by 5, the first month gets an extra day and becomes a 30-day month.
This does alright, but it does tend to drift relative to both the moon and the sun. So every 220 years there is an exceptional year that corrects the drift and gets things back on track. If the year is divisible by 220 it is the last year in the 11-year cycle and so would normally be a 12-month year. But since this is the exception, it is actually a 13-month year. It is also divisible by 5, so it should be a leap year, but it actually is not.
If you like numbers, this gives you a 220 year cycle containing 80,353 days, for an average of 365.2409 days per year, pretty close to the actual mean tropical year. Those 80,353 days are divided into 2721 months, making an average month of 29.5307 days, pretty close to a synodic month. It will take over 750 years to lose a day to the sun, and over 10,000 before it gains a day on the moon.
The origin date for the calendar is pretty arbitrary. I wanted the year to start around the Vernal equinox. That always seemed more like a beginning to me than the middle of the winter. I found that on March 20th, 1939 the equinox and new moon fell pretty close to each other, so I chose that as the origin date for my calendar. It's plenty early to cover all the dates in my life, so that suits me just fine.
My wife wanted to have the months named, so after thinking about it for a while, we decided to name then after good qualities that people have. I was born in the month Inventive, when were you born?