using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        foreach(var year in Enumerable.Range(2017, 9))

        {  

            var existingPlanStartDate = new DateTime(year, 1, 1);

            var existingPlanEndDate = existingPlanStartDate.AddMonths(12).AddSeconds(-1);

            var existingPlanLength = (existingPlanEndDate - existingPlanStartDate).TotalSeconds;

            var existingPlan = new SamplePlan() { StartDate = existingPlanStartDate, EndDate = existingPlanEndDate };

            var eventStart = existingPlanStartDate;

            while (eventStart.AddDays(7) <= existingPlanEndDate)

            {

                existingPlan.AddToEvents(eventStart, eventStart.AddDays(7));

                eventStart = eventStart.AddMonths(1);

            }

            Console.WriteLine("Existing Plan: {0}", existingPlan);

​

            var newPlanStartDate = existingPlan.EndDate.Date.AddDays(1);

            var newPlanEndDate = newPlanStartDate.AddSeconds(existingPlanLength);

            var leapDayDifference = newPlanStartDate.LeapDayCount(newPlanEndDate) - existingPlan.StartDate.LeapDayCount(existingPlan.EndDate);

            newPlanEndDate = newPlanEndDate.AddDays(leapDayDifference);

            var newPlan = new SamplePlan() { StartDate = newPlanStartDate, EndDate = newPlanEndDate };

            Console.WriteLine("New Plan: {0}", newPlan);

​

            Console.WriteLine();

        }

    }

}

​

public class SamplePlan

{

    public DateTime StartDate { get; set; }

    public DateTime EndDate { get; set; }

    private List<SamplePlanEvent> _events = new List<SamplePlanEvent>();

    public IEnumerable<SamplePlanEvent> Events { get { return (_events ?? new List<SamplePlanEvent>()).AsEnumerable(); } }

    public void AddToEvents(DateTime start, DateTime end)

    {

        AddToEvents(new SamplePlanEvent() { Start = start, End = end });

    }

    public void AddToEvents(SamplePlanEvent newEvent)

    {

        AddToEvents(new SamplePlanEvent[] { newEvent });

    }

    public void AddToEvents(IEnumerable<SamplePlanEvent> newEvents)

    {

        if (_events == null)

        {

            _events = new List<SamplePlanEvent>();

        }

        newEvents.ToList().ForEach(x => CheckEvent(x));

        _events.AddRange(newEvents);

    }   

    private void CheckEvent(SamplePlanEvent samplePlanEvent) 

    {

        if (samplePlanEvent.Start < StartDate) 

        {

            throw new ArgumentException("Start of event cannot be before start of plan", nameof(samplePlanEvent));

        }

        if (samplePlanEvent.End > EndDate) 

        {

            throw new ArgumentException("End of event cannot be after end of plan", nameof(samplePlanEvent));

        }

        if (samplePlanEvent.Start >= samplePlanEvent.End)

        {

            throw new ArgumentException("End of event cannot be before the start", nameof(samplePlanEvent));

        }

    }

    public override string ToString() 

    {

        var text = string.Format("{0} to {1}: ({2:0} days)", StartDate.ToString(Extensions.DATETIME_FORMAT), EndDate.ToString(Extensions.DATETIME_FORMAT), (EndDate - StartDate).TotalDays);

        if (Events != null && Events.Any())

        {

            text += Environment.NewLine + string.Join(Environment.NewLine, Events.OrderBy(x => x.Start).Select((x, y) => x.ToString()));

        }   

        return text;

    }

}

​

public class SamplePlanEvent

{

    public DateTime Start { get; set; }

    public DateTime End { get; set; }

    public override string ToString() => string.Format("{0} to {1}", Start.ToString(Extensions.DATETIME_FORMAT), End.ToString(Extensions.DATETIME_FORMAT));

}

​

public static class Extensions

{

    public static int LeapDayCount(this DateTime dateFrom, DateTime dateTo)

    {

        var start = (dateFrom <= dateTo ? dateFrom : dateTo).Date;

        var end = (dateFrom > dateTo ? dateFrom : dateTo).Date;

        var days = (end - start).Days + 1;

        return Enumerable.Range(0, days).Select(x => start.AddDays(x)).Count(x => x.Month == 2 && x.Day == 29);

    }

​

    public const string DATETIME_FORMAT = "dd-MMM-yy HH:mm:ss";

}