Working with Unix epoch in .NET (changing between DateTime and time since epoch)

private static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1);
        const double MicrosecondsInAMillisecond = 1e3;
 
        public static ulong ToHawkTime(this DateTime value)
        {
            double d = (value - UnixEpoch).TotalMilliseconds * MicrosecondsInAMillisecond;
            return (ulong) d;
        }
 
        public static DateTime ConvertFromHawkTime(this ulong value)
        {
            DateTime d = UnixEpoch + TimeSpan.FromMilliseconds(value/MicrosecondsInAMillisecond);
            return d;
        }