ios 获取当前系统时间

1. NSDate + NSDateFormatter

    NSDate *date = [NSDate date];
    NSDateFormatter *format = [[NSDateFormatter alloc] init];
    [format setDateFormat:@"YYYY/MM/dd hh:mm:ss.SSSSSSSS"];
    NSString *sDate = [format stringFromDate:date];
    NSLog(@"%@",sDate);

执行结果：精确到1ms

2015-03-30 21:35:13.799 My Legend[1037:53843] 2015/03/30 09:35:13.79900000

2.NSDate + NSCalendar + NSDateComponents

    NSDate *now = [NSDate date];
    NSLog(@"now date is: %@", now);
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSUInteger unitFlags = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond |NSCalendarUnitNanosecond;
    NSDateComponents *dateComponent = [calendar components:unitFlags fromDate:now];
    NSLog(@"%ld/%ld/%ld %ld:%ld:%ld.%ld",(long)dateComponent.year, (long)dateComponent.month, (long)dateComponent.minute, (long)dateComponent.hour, (long)dateComponent.minute, (long)dateComponent.second, (long)dateComponent.nanosecond);

执行结果：精确到1ns

2015-03-30 21:35:13.800 My Legend[1037:53843] now date is: 2015-03-30 13:35:13 +0000

2015-03-30 21:35:13.800 My Legend[1037:53843] 2015/3/35 21:35:13.800027012

3.C标准库函数 time

time_t 类型是用来存储从1970年到现在经过了多少秒

        time_t tBuf;
        tBuf=time(NULL);
        NSLog(@"%s", ctime(&tBuf));
        NSString *date = [NSString stringWithCString:ctime(&tBuf) encoding:NSUTF8StringEncoding];
        NSLog(@"%@\n", date);

执行结果 精确到1s

Sun Aug  2 11:42:01 2015

Sun Aug  2 11:42:01 2015

注： 可以用tm提取time时间信息

struct tm {//tm结构
	int	tm_sec;		/* seconds after the minute [0-60] */
	int	tm_min;		/* minutes after the hour [0-59] */
	int	tm_hour;	/* hours since midnight [0-23] */
	int	tm_mday;	/* day of the month [1-31] */
	int	tm_mon;		/* months since January [0-11] */
	int	tm_year;	/* years since 1900 */
	int	tm_wday;	/* days since Sunday [0-6] */
	int	tm_yday;	/* days since January 1 [0-365] */
	int	tm_isdst;	/* Daylight Savings Time flag */
	long	tm_gmtoff;	/* offset from CUT in seconds */
	char	*tm_zone;	/* timezone abbreviation */
};

        //提取从time获取的 time_t类型 时间信息
        struct tm tmBuf;
        tmBuf=*localtime(&tBuf);
        NSLog(@"%d:%d:%d", tmBuf.tm_hour, tmBuf.tm_min,tmBuf.tm_sec);

执行结果

11:57:27