def decade(date)
      d = ((date.year.abs.to_i / 10) * 10).to_s + 's'
      return [d, BCE_SUFFIX].join(' ') if (date.year <= -1)
      d
    end
def decade(date)
      d = ((date.year.abs.to_i / 10) * 10).to_s + 's'
      return [d, BCE_SUFFIX].join(' ') if (date.year <= -1)
      d
    end
Formats a CarbonDate::Date with decade precision
 Returns:
 A human-readable string representing the Date