# File lib/ice_cube/validations/monthly_interval.rb, line 31
      def validate(step_time, start_time)
        t0, t1 = start_time, step_time
        months = (t1.month - t0.month) +
                 (t1.year - t0.year) * 12
        offset = (months % interval).nonzero?
        interval - offset if offset
      end