Just ask Google:
public static DateTime GetInternetTime()
{
HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create("http://www.Google.com");
WebResponse response = myHttpWebRequest.GetResponse();
string todaysDates = response.Headers["date"];
return DateTime.ParseExact(todaysDates,
"ddd, dd MMM yyyy HH:mm:ss 'GMT'",
CultureInfo.InvariantCulture.DateTimeFormat,
DateTimeStyles.AssumeUniversal);
}