using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Availability> availability = new List<Availability>

            {

                new Availability { StartDate = new DateTime(2015,1,5), EndDate = new DateTime(2015,1,10)},

                new Availability { StartDate = new DateTime(2015,1,15), EndDate = new DateTime(2015,1,20)},

                new Availability { StartDate = new DateTime(2015,1,22), EndDate = new DateTime(2015,1,28)}

            };

             DateTime startDate = new DateTime(2015, 1, 1);

             DateTime endDate = new DateTime(2015, 1, 30);

​

             int totalDays = (int)(endDate - startDate).TotalDays + 1;

             availability.Add(new Availability { StartDate = endDate, EndDate = endDate });

​

            var result = from x in Enumerable.Range(0, totalDays)

            let d = startDate.AddDays(x)

            from a in availability.Select((v, i) => new { Value = v, Index = i })

            where (a.Index == availability.Count - 1 ? 

                     d <= a.Value.StartDate : d < a.Value.StartDate)

            && (a.Index != 0 ? d > availability[a.Index - 1].EndDate : true)

            group new { d, a } by a.Value.StartDate into g

            select new

            {

                 AvailableDates = String.Format("{0} - {1}",g.Min(x => x.d),

                                                            g.Max(x => x.d))

            };

           foreach(var item in result)

           {

               Console.WriteLine(item.AvailableDates);

           }

    }

}

​

 public class Availability

    {

        public DateTime StartDate { get; set; }

        public DateTime EndDate { get; set; }

    }