date-difference/index.js

Calculates the difference between two dates

'use strict'

/*
The formatDate function outputs the Years, Days, Hours, Minutes,
Seconds, and Milliseconds
*/
function formatDate(ms) {
  if (typeof ms !== 'number') {
    throw new TypeError('Expected a number');
  }

  return {
    years: Math.floor(ms / 31536000000),
    days: Math.floor(ms / 86400000 % 365),
    hours: Math.floor(ms / 3600000 % 24),
    minutes: Math.floor(ms / 60000 % 60),
    seconds: Math.floor(ms / 1000 % 60),
    milliseconds: Math.floor(ms % 1000)
  };
}

/*
Simple function to add a key value pair if the value is greater than 0
*/
function add(key, value, postfix) {
  if (value > 0) {
    key.push(value + postfix);
  }

  return key;
}

/*
The datePrettyFormat function converts milliseconds to a human readible
date output that corresponds to a specific format.
*/
function datePrettyFormat(ms, opts) {
  if (typeof ms !== 'number') {
    throw new TypeError('Expected a number');
  }

  if (ms < 1000) {
    return Math.ceil(ms) + 'ms';
  }

  opts = opts || {};

  var secDecimalDigits = typeof opts.secDecimalDigits === 'number' ? opts.secDecimalDigits : 1;
  var ret = [];
  var parsed = formatDate(ms);

  ret = add(ret, parsed.years, 'y');
  ret = add(ret, parsed.days, 'd');
  ret = add(ret, parsed.hours, 'h');
  ret = add(ret, parsed.minutes, 'm');

  if (opts.compact) {
    ret = add(ret, parsed.seconds, 's');
    return '~' + ret[0];
  }

  ret = add(ret, (ms / 1000 % 60).toFixed(secDecimalDigits).replace(/\.0$/, ''), 's');

  return ret.join(' ');
};

/*
Most of the solutions don't take into account a case that fails when
the two dates involved go across a daylight saving change. In this case,
the date on which day light saving change happens will have a duration in
milliseconds which != 1000*60*60*24, so the typical calculation will fail.

This function returns the difference between two dates represented in
a specific format

An example output without options is: 34y 338d 4h 8m 7s
An example output with options is: ~34y
*/
module.exports = function(a, b, opts) {
  // UTC time never observes Daylight Savings Time (DST)
  var utc1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate(), a.getHours(), a.getMinutes(), a.getSeconds(), a.getMilliseconds());
  var utc2 = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate(), b.getHours(), b.getMinutes(), b.getSeconds(), b.getMilliseconds());

  return datePrettyFormat((utc2 - utc1), opts);
}