Julian Day Calculator

is a 

whose Julian Day Number is 

Calendar: Auto (Julian/Gregorian)
Julian Only   
  1. All fields are editable. Change civil date to calculate Julian Day Number and day-of-week, or vice versa.
  2. After changing a text box, click or tap outside of the calculator, or press tab, to update all other data. (Don’t press enter key.)
  3. In Auto mode, civil calendar changes from Julian to Gregorian between October 5/16, 1582. For Julian civil dates after that date, as in England and colonies until 1753, select Julian Only.
  4. Courtesy of Numerical Recipes Software, 1988.