using System;

public class Program

{

    public static void Main()

    {

        String aLocalATLTime = "2021-07-14T13:59:01";

        DateTime dateTime = DateTime.ParseExact(aLocalATLTime, "yyyy-MM-ddTHH:mm:ss",

                                       System.Globalization.CultureInfo.InvariantCulture);

        TimeZoneInfo tzi = TimeZoneInfo.FindSystemTimeZoneById("US Eastern Standard Time");

        TimeSpan offset = tzi.GetUtcOffset(dateTime);

​

        Console.WriteLine(offset);

​

        Console.WriteLine(aLocalATLTime + offset.ToString().Substring(0, 3));

        Console.WriteLine("Call function " + addZoneToLocalTime(aLocalATLTime, "US Eastern Standard Time"));

    }

    public static String addZoneToLocalTime (string localTimeNoZone, string zoneName){

        DateTime dateTime = DateTime.ParseExact(localTimeNoZone, "yyyy-MM-ddTHH:mm:ss",

                                       System.Globalization.CultureInfo.InvariantCulture);

        TimeZoneInfo tzi = TimeZoneInfo.FindSystemTimeZoneById(zoneName);

        TimeSpan offset = tzi.GetUtcOffset(dateTime);

​

        return localTimeNoZone + offset.ToString().Substring(0, 3);

    }

}