public BillingCycle CalculateNext()
 {
     return(new BillingCycle(
                start:          this.end,
                end:            interval.CalculateNext(this.End),
                interval:       interval
                ));
 }
 public static BillingCycle CalculateFirst(DateTime start, Interval interval)
 {
     return(new BillingCycle(start, end: interval.CalculateNext(start), interval: interval));
 }