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