Get the number of days since the beginning of the year.

days_passed.js

// Days Passed Since New Year
// ==========================
//
// (c) 2013 Chao Huang <chao@realguess.net>

// Quick Example
// -------------
function days() {
  return (new Date(new Date().toISOString().substr(0,10)).getTime() - new Date('' + new Date().getFullYear()).getTime()) / (1000 * (60 * 60 * 24));
}
console.log(days());

// Detail Explanation
// ------------------
(function () {
  var seconds = 60 * 60 * 24; // seconds in a day
  var day1    = new Date('' + new Date().getFullYear());
  var today   = new Date(new Date().toISOString().substr(0,10));

  console.log((today.getTime() - day1.getTime()) / (1000 * seconds));
}).call(this);