ref: https://twitter.com/TUSBtatuharu/status/975705399570526213

Parking.cs

using System;

namespace Parking
{
    class Program
    {
        static readonly TimeSpan Delay = new TimeSpan(4, 0, 0);

        static readonly TimeSpan Span = new TimeSpan(9, 0, 0);

        static readonly TimeSpan OneDay = new TimeSpan(1, 0, 0, 0);

        static void Main(string[] args)
        {
            var intime = new DateTime(2018, 1, 1, 0, 0, 0);
            var outtime = new DateTime(2018, 1, 5, 21, 30, 0);
            Console.WriteLine($"{intime} ~ {outtime} -> {GetNightTime(intime, outtime)}");
        }

        static TimeSpan GetNightTime(DateTime intime, DateTime outtime)
        {
            if (intime <= outtime)
            {
                intime += Delay;
                intime = intime.TimeOfDay > Span ? intime.Date.AddDays(1.0) : intime;
                outtime += Delay;
                outtime = outtime.TimeOfDay > Span ? outtime.Date.AddDays(1.0) : outtime;
                return Span - intime.TimeOfDay + (outtime.Date - intime.Date - OneDay).Days * Span + outtime.TimeOfDay;
            }
            else
            {
                throw new ArgumentException();
            }
        }
    }
}