//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

（1）

    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];

   （2）NSString* date;

          NSDateFormatter* formatter = [[NSDateFormatteralloc]init];

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

           date = [formatterstringFromDate:[NSDatedate]];

           date 显示为 2011-11-01%2012:12:12

            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:-5*3600*24];//这是用现在的时间，往前面减5天，得到的时间。

 

    2. 从字符串来获得NSDate

    string strYear="1988";

    string strMonth="09";

    string strDay="18";

    string strHour="5";

    string strMinutes="18";

    string strSec="20";

    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];

    

    3. 从GMT时间，得到本地时间

    NSDate * oldDate=[dateformatter dateFromString:morelocationString];

 
    NSTimeInterval timeZoneOffset=[[NSTimeZonesystemTimeZone] secondsFromGMT];

    NSDate * newDate=[oldDate dateByAddingTimeInterval:timeZoneOffset];