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

The following leap day is really on February 29th, 2016 (this year). 
This implies we need to include an additional day toward the end of February every 4 years. 
As you might know, the 28th is generally the most recent day of February, however because of the leap day, we add an additional day to make the general days of one year 366 rather than the standard 365. 
During the current year (2016) the 100th day of the year is on April ninth. 
On a 365 days calander, the 100th day of the year is on April tenth.