//返回当前时间，精确到毫秒。
- (NSString *)getTimeNow

{

    NSString* date;

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

    //[formatter setDateFormat:@"YYYY.MM.dd.hh.mm.ss"];

    [formatter setDateFormat:@"YYYY-MM-dd hh:mm:ss:SSS"];

    date = [formatter stringFromDate:[NSDate date]];

    timeNow = [[NSString alloc] initWithFormat:@"%@", date];

    NSLog(@"%@", timeNow);

    return timeNow;

}

//获得系统时间

    NSDate * senddate=[NSDate date];

    NSDateFormatter *dateformatter=[[NSDateFormatteralloc] init];

    [dateformatter setDateFormat:@"HH:mm"];

    NSString * locationString=[dateformatter stringFromDate:senddate];

    //[dateformatter setDateFormat:@"YYYY-MM-dd-HH-mm-ss"];

    //NSString * morelocationString=[dateformatter stringFromDate:senddate];

    //获得系统日期

（）

    NSCalendar * cal=[NSCalendarcurrentCalendar];

    NSUInteger unitFlags=NSDayCalendarUnit|NSMonthCalendarUnit|NSYearCalendarUnit;

    NSDateComponents * conponent= [cal components:unitFlags fromDate:senddate];

    NSInteger year=[conponent year];

    NSInteger month=[conponent month];

    NSInteger day=[conponent day];

    NSString * nsDateString= [NSString stringWithFormat:@"M年-月-日",year,month,day]; 

    [dateformatter release];

   （）NSString* date;

          NSDateFormatter* formatter = [[NSDateFormatteralloc]init];

          [formattersetDateFormat:@"YYYY-MM-dd%20hh:mm:ss"];

           date = [formatterstringFromDate:[NSDatedate]];

           date 显示为 --%::

            想实现查找几天前的时间，可以用这个方法

         NSDate* date = [[NSDate alloc] init];

         date = [date dateByAddingTimeInterval:-**];//这是用现在的时间，往前面减5天，得到的时间。

    . 从字符串来获得NSDate

    string strYear="";

    string strMonth="";

    string strDay="";

    string strHour="";

    string strMinutes="";

    string strSec="";

    morelocationString=[NSString stringWithFormat:@"%s-%s-%s-%s-%s-%s",strYear.c_str(),strMonth.c_str(),

                        strDay.c_str(),strHour.c_str(),strMinutes.c_str(),strSec.c_str()];

    //根据时间字符串获得NSDate

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

    [dateformatter setDateFormat:@"YYYY-MM-dd-HH-mm-ss"];

    NSDate * oldDate=[dateformatter dateFromString:morelocationString];

    . 从GMT时间，得到本地时间

    NSDate * oldDate=[dateformatter dateFromString:morelocationString];

    NSTimeInterval timeZoneOffset=[[NSTimeZonesystemTimeZone] secondsFromGMT];

    NSDate * newDate=[oldDate dateByAddingTimeInterval:timeZoneOffset];