How do I convert a decimal year value into a Date in Javascript?

Question

OK, this is basically a Javascript version of How can I convert a decimal year value into a Date in Ruby? and not exactly a duplicate of Javascript function to conv

Answer 1

function leapYear (year){
  return ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
}

function getMonthAndDayFromDayOfYear(dayOfYear, year){
  var daysInMonthArray = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
  if (leapYear(year)) { daysInMonthArray[2] = 29; }

  var daysLeft = dayOfYear;
  var month = 0;
  for (i=0; i daysInThisMonth) {
      month += 1;
      daysLeft -= daysInThisMonth;
    } else {
      break;
    }
  }
  return { month: month, day: daysLeft };
}

function convertDecimalDate(decimalDate){
  decimalDate = parseFloat(decimalDate);
  var year = parseInt(decimalDate); // Get just the integer part for the year
  var daysPerYear = leapYear(year) ? 366 : 365; // Set days per year based on leap year or not
  var decimalYear = decimalDate - year; // A decimal representing portion of the year left
  var dayOfYear = Math.ceil(decimalYear * daysPerYear); // day of Year: 1 to 355 (or 366)
  var md = getMonthAndDayFromDayOfYear(dayOfYear, year);
  var day = md['day'];
  var month = md['month'];
  return new Date(year,month,day);
}

var date = convertDecimalDate(2015.0596924);