-(NSString*)changeNumToTime:(NSString*)str {

NSDate *date = [NSDate dateWithTimeIntervalSince1970:[str integerValue] - 8*3600]; //有八小时时差，需要减去

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];

[dateFormatter setDateFormat: @"yyyy-MM-dd HH:mm:ss"];

NSString *showtime = [dateFormatter stringFromDate:date];

return showtime;

}

　　eg: str = @"1441047302"时，返回的showtime = @"1970-01-18 00:17:27"。