function days_passed(dt) {  
  var current = new Date(dt.getTime());  
  var previous = new Date(dt.getFullYear(), 0, 1);  
  
  return Math.ceil((current - previous + 1) / 86400000);  
}  
console.log(days_passed(new Date(2015, 0, 15)));  
console.log(days_passed(new Date(2015, 11, 14)));