Unix time stamp, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            System.Globalization.CultureInfo customCulture = (System.Globalization.CultureInfo)System.Threading.Thread.CurrentThread.CurrentCulture.Clone();
            customCulture.NumberFormat.NumberDecimalSeparator = ".";
            System.Threading.Thread.CurrentThread.CurrentCulture = customCulture;
            Console.WriteLine("Hello, world!");
            double productExp = 24;
            DateTime expireDt = System.DateTime.UtcNow.AddHours(productExp);
            var expireTime = ConvertToUnixTimestamp(expireDt);
            Console.WriteLine(expireTime);
        }
        
        public static double ConvertToUnixTimestamp(DateTime gettime)
        {
            //create Timespan by subtracting the value provided from the Unix Epoch
            TimeSpan span = (gettime - new DateTime(1970, 1, 1, 0, 0, 0, 0).ToLocalTime());
            Console.WriteLine(span);
           //return the total seconds (which is a UNIX timestamp)
            return (double)span.TotalSeconds;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 11 Mb, average nr of threads: 3