If no arguments are supplied, the current month will be displayed. If only the -j flag is passed as an argument, the current month will be displayed with julian day numbering (see below).
NOTE: dates must be fully defined. Entering a year of 99 means 99 AD, not 1999 AD.
March 1999 Su M Tu W Th F Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
It takes into account the conversion from the Julian Calendar to the Gregorian Calendar in September, 1752. Note that this was the date used in the UK and all of her colonies (including the U.S.). Thus, in September, 1752, there were only 19 days. Eleven days had to be removed from the calendar to make up for inaccuracies in the Julian Calendar. So, September 2, 1752 was followed by September 14, 1752 in the UK and all colonies, as shown below.
September 1752 Su M Tu W Th F Sa 1 2 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
The Julian calendar has a leap year every 4 years. The Gregorian calendar has a leap year based on the following: If the year is evenly divisible by 400, it is a leap year, else, if the year is evenly divisible by 4, and not evenly divisible by 100, it is a leap year. All other years are not leap years. There is some contention over whether 4 AD was a leap year or not. Not all the experts agree, as there was a counting error in the beginning of the Julian calendar with regards to leap years that had to be corrected, and thus some years that would normally have been leap years were not. This calendar assumes that 4 AD was indeed a leap year.
URL: http://www.pip.dknet.dk/~c-t/calendar.html