Time windows in C# - A time window is a constrained period of time.

_Example.cs

public class Main
{
	public Main()
	{
		var windows = new List<TimeWindow>()
		{
			new TimeWindow(startHour: 0, endHour: 4, day: DayOfWeek.Monday),
			new TimeWindow(startHour: 0, endHour: 5, day: DayOfWeek.Thursday)
		};

		//check if we are in a window at the moment...
		if (windows.Any(x => x.IsInWindow(DateTime.Now)))
		{
			//do something..
		}

		//a timespan till the closest window..
		TimeSpan timeSpanTillClosestTimeWindow = windows.TimeSpanTillClosestTimeWindow();
	}
}

SystemTime.cs

public class SystemTime
{
	private static Func<DateTime> now = () => DateTime.Now;

	public static Func<DateTime> Now
	{
		get { return now; }
		set { now = value; }
	}
}

TimeWindow.cs

public class TimeWindow
{
	public int StartHour { get; private set; }
	public int EndHour { get; private set; }
	public DayOfWeek Day { get; private set; }

	public TimeWindow(int startHour, int endHour, DayOfWeek day)
	{
		if (startHour > endHour)
		{
			throw new ArgumentException("startHour can't be greater then endHour!");
		}

		StartHour = startHour;
		EndHour = endHour;
		Day = day;
	}

	public bool IsInWindow(DateTime dt)
	{
		return dt.DayOfWeek == Day && dt.Hour >= StartHour && dt.Hour < EndHour;
	}
}

TimeWindowExtensions.cs

public static class TimeWindowExtensions
{
	public static TimeSpan TimeSpanTillClosestTimeWindow(this IEnumerable<TimeWindow> timeWindows)
	{
		if (timeWindows == null)
		{
			throw new ArgumentNullException("timeWindows");
		}

		if (!timeWindows.Any())
		{
			throw new ArgumentException("there should be atleast one timewindow!");
		}

		var now = SystemTime.Now();
		var timeWindowsToday = timeWindows.Where(x => x.Day == now.DayOfWeek && x.StartHour - now.Hour > 0);

		if (!timeWindowsToday.Any())
		{
			return FindRelativeClosestDayFromToday(timeWindows, now);
		}

		var closestTimeWindows = from n in timeWindowsToday
								 let relativeClosestHour = n.StartHour - now.Hour
								 orderby relativeClosestHour
								 select n;


		var closestTimeWindow = closestTimeWindows.FirstOrDefault();
		return TimeSpanTillStart(0, closestTimeWindow.StartHour, now);
	}

	private static TimeSpan FindRelativeClosestDayFromToday(IEnumerable<TimeWindow> timeWindows, DateTime now)
	{
		var absoluteClosestDay = from n in timeWindows
						  let deltaDayOfWeek = n.Day - now.DayOfWeek
						  where deltaDayOfWeek > 0
						  orderby deltaDayOfWeek
						  select n;

		if (!absoluteClosestDay.Any())
		{
			return FindRelativeClosestDayNextWeek(timeWindows, now);
		}

		var closestDay = absoluteClosestDay.FirstOrDefault();
		var startOverInDays = closestDay.Day - now.DayOfWeek;
		return TimeSpanTillStart(startOverInDays, closestDay.StartHour, now);
	}

	private static TimeSpan TimeSpanTillStart(int startOverInDays, int startHour, DateTime now)
	{
		var dt = now;

		if (startOverInDays > 0)
		{
			dt = now.AddDays(startOverInDays);
		}

		var rdt = new DateTime(dt.Year, dt.Month, dt.Day, startHour, 0, 0);

		return rdt.Subtract(now);
	}

	private static TimeSpan FindRelativeClosestDayNextWeek(IEnumerable<TimeWindow> timeWindows, DateTime now)
	{
		var relativeClosestDays = from n in timeWindows
								  let deltaDayOfWeek = Math.Abs(n.Day - now.DayOfWeek)
								  orderby deltaDayOfWeek descending
								  select n;

		var relativeClosestDay = relativeClosestDays.FirstOrDefault();
		var startOverInDays = 7 - Math.Abs(relativeClosestDay.Day - now.DayOfWeek);
		return TimeSpanTillStart(startOverInDays, relativeClosestDay.StartHour, now);
	}
}