def brute_frequency(year=2017)

      FREQUENCY_CACHE["#{to_cron_s}|#{year}"] ||=
        begin

          deltas = []

          t = EtOrbi.make_time("#{year}-01-01") - 1
          t0 = nil
          t1 = nil

          loop do
            t1 = next_time(t)
            deltas << (t1 - t).to_i if t0
            t0 ||= t1
            break if deltas.any? && t1.year > year
            break if t1.year - t0.year > 7
            t = t1
          end

          Frequency.new(deltas, t1 - t0)
        end
    end
Mostly used as a #next_time sanity check.
Avoid for "business" use, it's slow.

2017 is a non leap year (though it is preceded by
a leap second on 2016-12-31)

Nota bene: cron with seconds are not supported.