const now = new Date()                                     // '2020-11-16 14:00'
const relative = formatRelativeTime('2020-11-19 12:00:00') // 'in 3 days'

function formatRelativeTime(date, reference = new Date(), language = navigator.language) {
  if (!date) return ''

  date = new Date(date)
  let delta = Math.round((date - reference) / 1000),
      deltaInUnit = delta,
      unit = 'second'

  const units = [
      { unit: 60, name: 'minute' },
      { unit: 60 * 60, name: 'hour' },
      { unit: 60 * 60 * 24, name: 'day' },
      { unit: 60 * 60 * 24 * 7, name: 'week' },
      { unit: 60 * 60 * 24 * 30, name: 'month' },
      { unit: 60 * 60 * 24 * 400, name: 'year' }
  ]

  for (let u of units) {
      if (Math.abs(delta) > u.unit) {
          deltaInUnit = delta / u.unit
          unit = u.name
      }
  }

  return new Intl.RelativeTimeFormat(language, {
      style: 'long',
      numeric: 'auto',
  }).format(deltaInUnit.toFixed(0), unit)
}