What is the 100th day of the calendar year(start at January1)?

The 100th day of the calendar year, starting from January 1, is April 10. In a standard year, January has 31 days, February has 28 days, March has 31 days, and the first 10 days of April total to 100 days. In a leap year, the 100th day would still fall on April 10.