code: function ago(amount, unit) {
    if (!isNumber(amount)) throw new Error("Usage ago(amount [,timeUnit])")
  
    return Date.now() - (+amount * unitToMillis(unit))
  }
docs: Return an epoch timestamp (millisecond) some amount of time ago. Accepts multiple
  units from milliseconds to years. Months and years are approximated.
  
  @param  {Number} amount Amount of time in `unit` default unit: milliseconds
  @param  {String} unit   Unit of time. If not provided, will be milliseconds
  ms, millis, millisecond, milliseconds
  s, sec, secs, second, seconds
  m, min, mins, minute, minutes
  h hr, hrs, hour, hours
  d, day, days
  w, wk, wks, week, weeks
  M, mon, mons, month, months (30.4375 days)
  q, qtr, qtrs, quarter, quarters
  y, yr, yrs, year, years (365.25 days)
  @return {Number}        Epoch (millisecond) time based on the input.