# File lib/ice_cube/validations/weekly_interval.rb, line 30
      def validate(step_time, start_time)
        return if step_time < start_time
        t0, t1 = start_time, step_time
        d0 = Date.new(t0.year, t0.month, t0.day)
        d1 = Date.new(t1.year, t1.month, t1.day)
        days = (d1 - TimeUtil.normalize_wday(d1.wday, week_start)) -
               (d0 - TimeUtil.normalize_wday(d0.wday, week_start))
        offset = ((days.to_i / 7) % interval).nonzero?
        (interval - offset) * 7 if offset
      end