In a common year, which consists of 365 days, September 11th is the 254th day. In a leap year, which has 366 days, it is the 255th day of the year.
The calculation for this ordinal date involves summing the number of days in each preceding month and adding the day number of the current month. For a common year, the total is derived from January (31), February (28), March (31), April (30), May (31), June (30), July (31), and August (31), plus the 11 days of September. The sum (31+28+31+30+31+30+31+31+11) equals 254. During a leap year, February contains 29 days, which increases the cumulative total by one, making September 11th the 255th day.
As a result of this calculation, there are consistently 111 days remaining in the year after September 11th. This figure remains the same for both common and leap years because the extra day in a leap year is accounted for in the ordinal day count (366 - 255 = 111) just as it is in a common year (365 - 254 = 111).
