JSFiddle - React, Tailwind, and code Playground

JavaScript

function dayofyear(d) {   // d is a Date object
    var yn = d.getFullYear();
    var mn = d.getMonth();
    var dn = d.getDate();
    var d1 = new Date(yn,0,1,12,0,0); // noon on Jan. 1
    var d2 = new Date(yn,mn,dn,12,0,0); // noon on input date
    var ddiff = Math.round((d2-d1)/864e5);
    return ddiff+1; 
}

function printDayOfYear(str) {
    var date = new Date(Date.parse(str));
    var dayOfYear = dayofyear(date);
    console.log(date + ': ' + dayOfYear);
}

function printDayOfYear2(date) {
    var dayOfYear = dayofyear(date);
    console.log(date + ': ' + dayOfYear);
}

printDayOfYear("2014-10-15");
printDayOfYear2(new Date(2014, 9, 15));