using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public void Main()

    {

        Dictionary<DateTimeOffset, List<DateTimeOffset>> availableSlots = new Dictionary<DateTimeOffset, List<DateTimeOffset>>();

        DateTime readyByTime = new DateTime(2020, 01, 14, 9, 0, 0);

        DateTime closeTime = new DateTime(2020, 01, 14, 21, 0, 0);

        DateTime latesBookingTime = new DateTime(2020, 01, 14, 15, 0, 0);

        var minHourWindow = 3;

        var customerLocalTime = DateTimeOffset.UtcNow;

        for (int i = readyByTime.Hour; i <= latesBookingTime.Hour; i++)

        {

            var nextReadyTime = new DateTimeOffset(customerLocalTime.Date.AddHours(i), customerLocalTime.Offset);

            availableSlots[nextReadyTime] = new List<DateTimeOffset>();

            for (int j = Math.Max(nextReadyTime.Hour + minHourWindow, this.Ceil(latesBookingTime, TimeSpan.FromHours(1)).Hour); j <= closeTime.Hour; j++)

            {

                var nextEndTime = new DateTimeOffset(customerLocalTime.Date.AddHours(j), customerLocalTime.Offset);

                availableSlots[nextReadyTime].Add(nextEndTime);

            }

        }

        foreach (KeyValuePair<DateTimeOffset, List<DateTimeOffset>> kvp in availableSlots)

        {

            foreach(var endTime in kvp.Value)

            {

                Console.WriteLine("Key = {0}, Value = {1}", kvp.Key.ToString(), endTime.ToString());

            }

        }

    }

    public DateTime Ceil(DateTime date, TimeSpan span)

    {

        long ticks = (date.Ticks + span.Ticks - 1) / span.Ticks;

        return new DateTime(ticks * span.Ticks);

    }

}