milliseconds-to-iso-8601-duration

milliseconds-to-iso-8601-duration is a tiny library to convert
millisecond durations to [ISO 8601][durations-1]
[durations][durations-2]. The existing [xsdurations][] is usually
sufficient for consuming ISO 8601 durations, so use that if you have a
duration and want to add it to a date. Here we are only concerned
with producing durations from dates. Instances of [Date][] support
[subtraction returning millisecond offsets][subtraction], so you can
use this library for things like:

var a = new Date(2015, 5, 21, 14, 11);
var b = new Date(2015, 5, 22, 16, 45);
var duration = millisecondsToISO8601Duration.iso8601duration(b - 1);

to set duration to "P1DT2H34M".

Tests
=====

The test suite uses [QUnit][] via [Grunt][] and [grunt-contrib-qunit].
Install the tools with [npm][]:

$ npm install -g grunt-cli
$ npm install

And then run the tests with:

$ grunt

[durations-1]: https://tools.ietf.org/html/rfc3339#page-13
[durations-2]: http://en.wikipedia.org/wiki/ISO_8601#Durations
[xsdurations]: https://github.com/revington/xsdurationjs
[Date]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date
[subtraction]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#Example:_Calculating_elapsed_time
[QUnit]: http://qunitjs.com/
[Grunt]: http://gruntjs.com/
[grunt-contrib-qunit]: https://github.com/gruntjs/grunt-contrib-qunit
[npm]: https://www.npmjs.com/