using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

​

        CalculateForVisits(new [] {

            //  Put your date ranges here

            //  <date>, number of days

            //  or <date>, <date>

​

            //  Calculations will be wrong if the date ranges overlap.

            new DateRange(new DateTime(2015, 5, 1), new DateTime(2015, 7, 20)),

            new DateRange(new DateTime(2016, 5, 1), 150),

        });

    }

    public static void CalculateForVisits(DateRange[] Visits)

    {

        var visitsStart = new DateTime(Visits.Select(v => v.Start.Ticks).Min());

        var visitsEnd = new DateTime(Visits.Select(v => v.End.Ticks).Max());

        var maximumOverlapDaysObserved = 0;

        DateRange maximumOverlapedDateRange = null;

        for(var periodStart = visitsStart; periodStart < visitsEnd; periodStart += TimeSpan.FromDays(1))

        {

            var period = new DateRange(periodStart, 365);

            var overlap = Visits.Select(v => v.DaysOverlapInclusive(period)).Sum();

            if (overlap > maximumOverlapDaysObserved)

            {

                maximumOverlapDaysObserved = overlap;

                maximumOverlapedDateRange = period;

            }

        }

        Console.WriteLine("Considering the following visits:");

        foreach(var visit in Visits)

            Console.WriteLine("  " + visit);

        Console.WriteLine();

        Console.WriteLine("The maximum number of days visited for any 365 day period is " + maximumOverlapDaysObserved + " during time period " + maximumOverlapedDateRange + ".");

        var initialYear = Visits.Select(v => v.Start.Year).Min();

        var lastYear = Visits.Select(v => v.End.Year).Max();

        for(var i = initialYear; i <= lastYear; i++) {

            var yearRange = new DateRange(new DateTime(i, 1, 1), new DateTime(i+1,1,1).AddDays(-1));

            var overlap = Visits.Select(v => v.DaysOverlapInclusive(yearRange)).Sum();

            Console.WriteLine("Total time spent in year " + i + " is " + overlap + " days.");

        }

    }                                

}

​

​

public class DateRange

{

    public readonly DateTime Start;

    public readonly DateTime End;

    public DateRange(DateTime start, DateTime end)

    {

        Start = start;

        End = end;

        if (End < Start)

            throw new ArgumentException("DateRange specified with start and end dates out of order.");

    }

    public DateRange(DateTime start, int totalDaysInclusive) : this(start, start + TimeSpan.FromDays(totalDaysInclusive - 1))

    {

    }

    public int DaysOverlapInclusive(DateRange other)

    {

        var overlapStart = new DateTime(Math.Max(this.Start.Ticks, other.Start.Ticks));

        var overlapEnd = new DateTime(Math.Min(this.End.Ticks, other.End.Ticks));

        if (overlapEnd < overlapStart)

            return 0;

        return (overlapEnd - overlapStart).Days + 1;

    }

    public override string ToString()

    {

        var durationInclusive = (End - Start).Days + 1;

        return Start.ToLongDateString() + " to " + End.ToLongDateString() + " (" + durationInclusive + " days)";

    }

}