// in miliseconds
var units = {
  year  : 24 * 60 * 60 * 1000 * 365,
  month : 24 * 60 * 60 * 1000 * 365/12,
  day   : 24 * 60 * 60 * 1000,
  hour  : 60 * 60 * 1000,
  minute: 60 * 1000,
  second: 1000
}

var rtf = new Intl.RelativeTimeFormat('en', { numeric: 'auto' })

var getRelativeTime = (d1, d2 = new Date()) => {
  var elapsed = d1 - d2

  // "Math.abs" accounts for both "past" & "future" scenarios
  for (var u in units) 
    if (Math.abs(elapsed) > units[u] || u == 'second') 
      return rtf.format(Math.round(elapsed/units[u]), u)
}

// test-list of dates to compare with current date
[
  '10/20/1984',
  '10/20/2015',
  +new Date() - units.year,
  +new Date() - units.month,
  +new Date() - units.day,
  +new Date() - units.hour,
  +new Date() - units.minute,
  +new Date() + units.minute*2,
  +new Date() + units.day*7,
]
.forEach(d => console.log(   
  new Date(d).toLocaleDateString(),
  new Date(d).toLocaleTimeString(), 
  '(Relative to now) →',
  getRelativeTime(+new Date(d))
))
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