# File lib/rufus/scheduler/zotime.rb, line 437
    def inc(t, dir)

      if t.is_a?(Numeric)
        nt = self.dup
        nt.seconds += dir * t.to_f
        nt
      elsif t.respond_to?(:to_f)
        @seconds + dir * t.to_f
      else
        fail ArgumentError.new(
          "cannot call ZoTime #- or #+ with arg of class #{t.class}")
      end
    end