julian

Convert between JavaScript's Date object and Julian dates used in astronomy and history.

usage

  var julian = require('julian');

  var now = new Date();           // Let's say it's Thu, 21 Nov 2013 10:47:02 GMT
  var jd = '';

  console.log(jd = julian(now));  // -> '2456617.949335'
  console.log(julian.toDate(jd)); // -> Timestamp above in local TZ

api

julian(date)

Convert a Date into a string representing Julian Date.

julian.toDate(jd)

Convert a Julian Date to a javascript Date.

julian.toJulianDay(date)

return a javascript Date or timestamp to the julian day. An integer day is returned.

julian.toMillisecondsInJulianDay (date)

returns the number of milliseconds since the start of the julian day. Note, the julian day starts at noon, not at midnight. This seems strange, if you don't have an accurate clock, then finding noon accurately is easy (from the sun) but finding midnight is not easy. Julian days have been used by astronomers since before accurate clocks.

julian.fromJulianDayAndMilliseconds(day, ms)

Converts a julian day and ms back to a javascript timestamp. Also, note that this is reversable with out floating point errors.

var date = Date.now()

assert.equal(
  julian.fromJulianDayAndMilliseconds(
    julian.toJulianDay(date),
    julian.toMillisecondsInJulianDay(date)
  ),
  date
)

notice

Date systems are a mess. Leap years, leap seconds, epochs, different calendars using the same nomenclature, different countries using different calendars at the same time, etc.

This library doesn't even try to cope with all that shit.

If you want to display calendar dates in format appropriate for a given culture at a given time in history - well, first of all, good luck to you. For example, the October Revolution took place in what most of us now call November. That's because in Tsar's Russia they still used obsolete Julian calendar until bolsheviks finally adopted Gregorian. Hey thanks, Lenin!

Luckily for historians and astronomers, they can just say the revolution began on 2421540 Julian Day, and that's the whole point. You don't mess with naming days, you just count them. Day 0 would be 1 January 4713 BC. Or is that 27 Nov 4714 BC? Fuck.

license

MIT

author

Stepan Stolyarov stepan.stolyarov@gmail.com