What is the 100th day of the calender starting at ( January 1)?

The 100th day of the calendar year, starting from January 1, is April 10. This is calculated by adding the days in January (31), February (28 in a common year), and March (31) to reach 90 days by the end of March, and then counting an additional 10 days into April. In a leap year, the 100th day would be April 9.