/// <summary>
    /// 時間幫助類
    /// </summary>
    public class TimeHelper
    {
        /// <summary>
        /// 日期轉(zhuan)換成unix時間(jian)戳
        /// </summary>
        /// <param name="dateTime"></param>
        /// <returns></returns>
        public static long DateTimeToUnixTimestamp(DateTime dateTime)
        {
            var start = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Local);
            return Convert.ToInt64((dateTime - start).TotalMilliseconds);
        }

        /// <summary>
        /// unix時間戳轉換成日期(qi)
        /// </summary>
        /// <param name="unixTimeStamp">時間戳(chuo)（秒）</param>
        /// <returns></returns>
        public static DateTime UnixTimestampToDateTime(DateTime target, long timestamp)
        {
            var start = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Local);
            return start.AddSeconds(timestamp);
        }

    }

TimeHelper.DateTimeToUnixTimestamp(DateTime.Now.ToUniversalTime());

    Date d1 = new Date();
    d1.setTime(1490232651510L);
    System.err.println(d1);