C# extension method to get HTTP formatted date-time string from DateTimeOffset

DateTimeOffsetExtensions.cs

namespace System
{
    /// <summary>
    /// Extensions to <see cref="DateTimeOffset"/>
    /// </summary>
    public static class DateTimeOffsetExtensions
    {
        /// <summary>
        /// Returns the localised date-time as GMT formatted for HTTP headers, e.g.
        ///   Thu, 09 Oct 2014 10:31:33 GMT
        /// </summary>
        public static string ToHttpHeaderString(this DateTimeOffset offset)
        {
            var gmt = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(offset, "GMT Standard Time");
            return string.Format("{0:ddd, dd MMM yyyy HH:mm:ss} GMT", gmt);
        }
    }
}

Uh... sorry, but no. "GMT Standard Time" is the time zone for London and the rest of the UK, which alternates between GMT (+00:00) and BST (+01:00) for daylight saving time. The correct time zone would just be "UTC".

However, this whole function can just be replaced by .ToString("R"). See The RFC1123 ("R", "r") Format Specifier.