// Retrieved from:
// https://github.com/azer/relative-date/blob/master/lib/relative-date.js.

var relativeDate = (function(undefined){

  var SECOND = 1000,
      MINUTE = 60 * SECOND,
      HOUR = 60 * MINUTE,
      DAY = 24 * HOUR,
      WEEK = 7 * DAY,
      YEAR = DAY * 365,
      MONTH = YEAR / 12;

  var formats = [
    [ 0.7 * MINUTE, 'just now' ],
    [ 1.5 * MINUTE, 'a minute ago' ],
    [ 60 * MINUTE, 'minutes ago', MINUTE ],
    [ 1.5 * HOUR, 'an hour ago' ],
    [ DAY, 'hours ago', HOUR ],
    [ 2 * DAY, 'yesterday' ],
    [ 7 * DAY, 'days ago', DAY ],
    [ 1.5 * WEEK, 'a week ago'],
    [ MONTH, 'weeks ago', WEEK ],
    [ 1.5 * MONTH, 'a month ago' ],
    [ YEAR, 'months ago', MONTH ],
    [ 1.5 * YEAR, 'a year ago' ],
    [ Number.MAX_VALUE, 'years ago', YEAR ]
  ];

  function relativeDate(input,reference){
    !reference && ( reference = (new Date).getTime() );
    reference instanceof Date && ( reference = reference.getTime() );
    input instanceof Date && ( input = input.getTime() );

    var delta = reference - input,
        format, i, len;

    for(i = -1, len=formats.length; ++i < len; ){
      format = formats[i];
      if(delta < format[0]){
          return format[2] == undefined ? format[1] : Math.round(delta/format[2]) + ' ' + format[1];
      }
    };
  }

  return relativeDate;

})();