DateTime - IsDaylightSavingTime

Returns a value indicating whether the specified date and time is within the specified daylight saving time period.

Try it

public static void Main()
{
    DateTime startDate = new DateTime(2018, 3, 11);
    DateTime endDate = new DateTime(2018, 11, 4);
    TimeSpan delta = startDate - endDate;

    DateTime date = new DateTime(2018, 8, 9);
    DaylightTime daylightTimes = new DaylightTime(startDate, endDate, delta);

    // C# Extension Method: DateTime  - IsDaylightSavingTime
    bool flag = date.IsDaylightSavingTime(daylightTimes);

    if (flag)
    {
        Console.WriteLine("{0} is a day light saving time.", date);
    }
}

View Source 
using System;
using System.Globalization;

public static partial class Extensions
{
    /// <summary>
    ///     Returns a value indicating whether the specified date and time is within the specified daylight saving time
    ///     period.
    /// </summary>
    /// <param name="time">A date and time.</param>
    /// <param name="daylightTimes">A daylight saving time period.</param>
    /// <returns>true if  is in ; otherwise, false.</returns>
    public static Boolean IsDaylightSavingTime(this DateTime time, DaylightTime daylightTimes)
    {
        return TimeZone.IsDaylightSavingTime(time, daylightTimes);
    }
}