1.NSDate 和 NSString 之间的转换

• 这之间的转换主要依靠NSDateFormatter
• NSDate <------> NSString

    NSDateFormatter *formatter = [[NSDateFormatter alloc]init];formatter.dateFormat = @"yyyy-MM-dd HH:mm:ss";NSString *strDate = [formatter stringFrom:date];NSDate *date =[formatter dateFrom:str];

其中dateFormat格式可以参考苹果官方文档Working With Fixed Format Date Representations

2.NSDate通过NSCalendar操作

• 转化为NSCalendar方便于获得各种日期参数

-(NSDateComponents*)getDateComponentsFromDate:(NSDate*)date{NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];//设置成中国阳历NSDateComponents *comps = [[NSDateComponents alloc] init];NSInteger unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSWeekdayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit;//这句我也不明白具体时用来做什么。。。comps = [calendar components:unitFlags fromDate:date];return comps;
}

• NSDateComponets 可以获得 年，月，日，以及第几个星期等等

3. NSDate之间的比较

• 一般用来做文件夹排序等操作

[_dateDirArray sortedArrayUsingComparator: ^NSComparisonResult(__nonnull id obj1,__nonnull id obj2){NSString *stringDateObj1 = obj1;NSString *stringDateObj2 = obj2;NSDate *dateObj1 = [dateFormatter dateFromString:stringDateObj1];NSDate *dateObj2  =[dateFormatter dateFromString:stringDateObj2];return [dateObj1 compare:dateObj2];}]

• 另外一种用来计算两者之间的时间差

 //lastDate和nowDate为NSDate类型，最后得到的秒数为lastDate－nowDate所得NSInteger timeDistance= [lastDate timeIntervalSinceDate:nowDate]

以及

    // 时间1NSDate *date1 = [NSDate date];NSTimeZone *zone1 = [NSTimeZone systemTimeZone];NSInteger interval1 = [zone1 secondsFromGMTForDate:date1];NSDate *localDate1 = [date1 dateByAddingTimeInterval:interval1];// 时间2NSDate *date2 = [NSDate date];NSTimeZone *zone2 = [NSTimeZone systemTimeZone];NSInteger interval2 = [zone2 secondsFromGMTForDate:date2];NSDate *localDate2 = [date2 dateByAddingTimeInterval:interval2];// 时间2与时间1之间的时间差（秒）double intervalTime = [localDate2 timeIntervalSinceReferenceDate] - [localDate1 timeIntervalSinceReferenceDate];NSInteger seconds = lTime % 60;NSInteger minutes = (lTime / 60) % 60;NSInteger hours = (lTime / 3600);NSInteger days = lTime/60/60/24;NSInteger month = lTime/60/60/24/12;NSInteger years = lTime/60/60/24/365;