long oldtime=1703235741;
System.DateTime startTime = TimeZone.CurrentTimeZone.ToLocalTime(new System.DateTime(1970, 1, 1, 0, 0, 0, 0));
var newtime=startTime.AddMilliseconds(oldtime).ToString("yyyy-MM-dd HH:mm:ss.fff");
DateTime time = DateTime.Now;
System.DateTime startTime = TimeZone.CurrentTimeZone.ToLocalTime(new System.DateTime(1970, 1, 1));//当地时区
TimeSpan ts = time - startTime;
var timestamp = Convert.ToInt64(ts.TotalSeconds);