One day it occurred to me that the Gregorian calendar everyone uses isn't quite accurate to the average solar/tropical year. Gregory averages out to 365.2425 days per year by the end of every 400th year, but the average solar/tropical year is actually 365.24219 days.
Also, it has always irked me that Gregory's months are not precisely four weeks in length and that every year and month begins on a different day of the week.
Therefore I created the Septem calendar which averages out to 365.2421875 days per year by the end of every 896th year. In addition, every month is four weeks in length and every year and month begins on First Day, aka Sunday. This is accomplished without any need to ever reset the progression of weekdays.
The Septem calendar began on Gregorian Sunday, September 23, 2007 AD. This was the Fall Equinox in the Northern Hemisphere and the Spring Equinox in Southern Hemisphere, and it took place in the Gregorian year I first came to a saving faith in Jesus Christ. (I needed a starting date for the calendar and that seemed as good as any. It's also around the time many harvest calendars begin.)
The calendar begins on year 1. Years previous to year 1 begin on -1 and proceed to -2 and so forth. There is no year 0. Assume year -1 was an 896th year with the Second Catch Week ignored (see below).
First Day (equivalent to Gregorian Sunday)
Each month has exactly 28 days (4 weeks), making the typical year 364 days in length.
At the end of every year evenly divisible by 7 (7, 14, 21, etc.) there occurs what is called a "Catch Week". This is a week that takes place after the Thirteenth Month and belongs to no particular month. Its days are numbered 1-7.
At the end of every year evenly divisible by 28 (28, 56, 84, etc.) there occurs a "First Catch Week" and then a "Second Catch Week" right after the first. As with the First Catch Week, the Second belongs to no particular month. Its days are numbered 1-7.
The Second Catch Week of every year evenly divisible by 896 is skipped despite it being a 28th year. It doesn't happen. The First Catch Week is simply a Catch Week, not bothering with the word "First".
Every 7th, 28th, and 896th year is called a "catch year".
THE END RESULT
At the end of the 896th year's Catch Week the amount of days in every previous year will average out to 365.2421875 days, just a tad under the average solar/tropical year of 365.24219 days.
An average solar/tropical year is 365 days, 5 hours, 48 minutes, and 45.216 seconds.
A Septem year is 365 days, 5 hours, 48 minutes, and 45 seconds. There's a difference of only 0.216 seconds with the average solar/tropical year!
The Gregorian year is 365 days, 5 hours, 49 minutes, and 12 seconds. There's a difference of 26.784 seconds with the average solar/tropical year.
WRITING THE DATE
First Day, 1st Month 1, 1. (1/1/1)
Second Day, 1st Catch Week 3, 7. (14/3/7)
Third Day, 2nd Catch Week 6, 28. (15/6/28)
Fourth Day, 13th Month 28, -1. (13/7/-1)
GREGORIAN TO SEPTEM CONVERSION
See this link.