What is the 100th day of the calendar year? (start at january 1st)

The next leap day is actually on February 29th, 2016 (this year). 
This means that we have to add an extra day at the end of February every 4 years.
As you may know, the 28th is usually the last day of February, but due to the leap day, we add an extra day to make the overall days of one year 366 instead of the usual 365.

For this year (2016) the 100th day of the year is on April 9th.
On a 365 days calendar, the 100th day of the year is on April 10th.