DateTimeHelpers.cs

    public static class DateTimeHelper
    {
        private static readonly DateTime unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);

        /// <summary>
        /// Return the number of seconds since Epoch (1st Jan 1970)
        /// </summary>
        /// <returns>Seconds since epoch</returns>
        public static long SecondsSinceEpoch()
        {
            TimeSpan t = DateTime.UtcNow - new DateTime(1970, 1, 1);
            int secondsSinceEpoch = (int)t.TotalSeconds;
            return secondsSinceEpoch;
        }

        public static DateTime FromUnixTime(long unixTime)
        {
            var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
            return epoch.AddSeconds(unixTime);
        }

        public static long ToUnixTime(DateTime dateTime)
        {
            return (long)(dateTime - unixEpoch).TotalSeconds;
        }

        public static long ToUnixTimeMilliseconds(DateTime dateTime)
        {
            return (long)(dateTime - unixEpoch).TotalMilliseconds;
        }
    }