字符串转日期：

long ltime = Convert.ToInt64("1481097219000");

System.DateTime startTime = TimeZone.CurrentTimeZone.ToLocalTime(new System.DateTime(1970, 1, 1)).ToLocalTime(); // 当地时区

DateTime dt = startTime.AddMilliseconds(ltime);

日期转 long

System.DateTime startTime = TimeZone.CurrentTimeZone.ToLocalTime(new System.DateTime(1970, 1, 1)); // 当地时区

long showTime = (long)(System.DateTime.Now - startTime).TotalMilliseconds;

时间戳计算