DateTime currentTime = new DateTime(2018, 1, 1, 11, 30, 0);
            DateTime StartTime = new DateTime(2019, 1, 14, 10, 30, 0);
            DateTime EndTime = new DateTime(2021, 1, 14, 18, 30, 0);


            Console.WriteLine("===============Before change :" + currentTime.IsPriorTo(StartTime));
            currentTime = currentTime.AddHours(-2);
            Console.WriteLine("===============After change :" + currentTime.IsPriorTo(StartTime));
　　　　　　　　
　　　　　　  public static bool IsPriorTo(this DateTime thisTime,DateTime compareTime)
        　　{
            　　return compareTime.TimeOfDay.CompareTo(thisTime.TimeOfDay) == 1;
        　　}

 　　　　public static bool IsInside(this DateTime thisTime,DateTime startTime,DateTime endTime)
        {
            return startTime.TimeOfDay.CompareTo(thisTime.TimeOfDay) == -1 && endTime.TimeOfDay.CompareTo(thisTime.TimeOfDay) == 1;
        }

        public static bool IsInside(this DateTime thisTime,DateTime startTime,DateTime endTime)
        {
            return startTime.IsPriorTo(thisTime) && endTime.IsPriorTo(thisTime) == false;
        }