how can I convert day of year to date in javascript?

Question

I want to take a day of the year and convert to an actual date using the Date object. Example: day 257 of 1929, how can I go about doing this?

Answer 1

Here's my implementation, which supports fractional days. The concept is simple: get the unix timestamp of midnight on the first day of the year, then multiply the desired day by the number of milliseconds in a day.

/**
 * Converts day of the year to a unix timestamp
 * @param {Number} dayOfYear 1-365, with support for floats
 * @param {Number} year (optional) 2 or 4 digit year representation. Defaults to
 * current year.
 * @return {Number} Unix timestamp (ms precision)
 */
function dayOfYearToTimestamp(dayOfYear, year) {
  year = year || (new Date()).getFullYear();
  var dayMS = 1000 * 60 * 60 * 24;

  // Note the Z, forcing this to UTC time.  Without this it would be a local time, which would have to be further adjusted to account for timezone.
  var yearStart = new Date('1/1/' + year + ' 0:0:0 Z');

  return yearStart + ((dayOfYear - 1) * dayMS);
}

// usage

// 2015-01-01T00:00:00.000Z
console.log(new Date(dayOfYearToTimestamp(1, 2015)));

// support for fractional day (for satellite TLE propagation, etc)
// 2015-06-29T12:19:03.437Z
console.log(new Date(dayOfYearToTimestamp(180.51323423, 2015)).toISOString);