What is the 100th day of the year starting at Janurary 1st?

The 100th day of the year, starting from January 1st, is April 10th in a common year. In a leap year, it falls on April 9th. This is because January has 31 days, February has 28 days (29 in a leap year), and March has 31 days, totaling 90 days by the end of March. Adding 10 days in April brings you to April 10th for common years.