using System;

using System.Collections.Generic;

​

public class Period

{

    public DateTime Begin { get; set; }

    public DateTime End { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        var periods = new List<Period>();

        var period = new Period { Begin = new DateTime(2017, 07, 01, 7, 50, 0), End = new DateTime(2017, 07, 01, 12, 30, 0) };        

        var previous = period.Begin;

​

        do

        {

            var next = new Period { End = previous.AddSeconds(3600 - previous.Minute * 60 - 1), Begin = previous };

            previous = next.End.AddSeconds(1);

            if (next.End < period.End.AddSeconds(-1))

                periods.Add(next);

            else

            {

                next.End = period.End.AddSeconds(-1);

                periods.Add(next);

                break;

            }                

        } while (true);

        periods.ForEach(x => Console.WriteLine("{0} - {1}", x.Begin, x.End));

    }

}