NSDate的运算

NSDate存储的是世界标准时(UTC)，输出时需要根据时区转换为本地时间 
Dates 
NSDate类提供了创建date，比较date以及计算两个date之间间隔的功能。Date对象是不可改变的。 
如果你要创建date对象并表示当前日期，你可以alloc一个NSDate对象并调用init初始化： 
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NSDate *now = [[NSDate alloc] init];

或者使用NSDate的date类方法来创建一个日期对象。如果你需要与当前日期不同的日期，你可以使用NSDate的initWithTimeInterval...或dateWithTimeInterval...方法，你也可以使用更复杂的calendar或date components对象。 
创建一定时间间隔的NSDate对象： 
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NSTimeInterval secondsPerDay = 24 * 60 * 60;

NSDate *tomorrow = [[NSDate alloc] initWithTimeIntervalSinceNow:secondsPerDay];

NSDate *yesterday = [[NSDate alloc] initWithTimeIntervalSinceNow:-secondsPerDay];

[tomorrow release]; 
[yesterday release];

使用增加时间间隔的方式来生成NSDate对象： 
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NSTimeInterval secondsPerDay = 24 * 60 * 60;

NSDate *today = [[NSDate alloc] init]; 
NSDate *tomorrow, *yesterday;

tomorrow = [today dateByAddingTimeInterval: secondsPerDay]; 
yesterday = [today dateByAddingTimeInterval: -secondsPerDay];

[today release];

如果要对NSDate对象进行比较，可以使用isEqualToDate:, compare:, laterDate:和 earlierDate:方法。这些方法都进行精确比较，也就是说这些方法会一直精确比较到NSDate对象中秒一级。例如，你可能比较两个日期，如果他们之间的间隔在一分钟之内则认为这两个日期是相等的。在这种情况下使用，timeIntervalSinceDate:方法来对两个日期进行比较。下面的代码进行了示例： 
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if (fabs([date2 timeIntervalSinceDate:date1]) < 60) ...

NSCalendar & NSDateComponents 
日历对象封装了对系统日期的计算，包括这一年开始，总天数以及划分。你将使用日历对象对绝对日期与date components（包括年，月，日，时，分，秒）进行转换。 
NSCalendar定义了不同的日历，包括佛教历，格里高利历等（这些都与系统提供的本地化设置相关）。NSCalendar与NSDateComponents对象紧密相关。 
你可以通过NSCalendar对象的currentCalendar方法来获得当前系统用户设置的日历。 
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NSCalendar *currentCalendar = [NSCalendar currentCalendar];

NSCalendar *japaneseCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSJapaneseCalendar];

NSCalendar *usersCalendar = [[NSLocale currentLocale] objectForKey:NSLocaleCalendar]; 
usersCalendar和currentCalendar对象是相等的，尽管他们是不同的对象。 
你可以使用NSDateComponents对象来表示一个日期对象的组件——例如年，月，日和小时。如果要使一个NSDateComponents对象有意义，你必须将其与一个日历对象相关联。下面的代码示例了如何创建一个NSDateComponents对象： 
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NSDateComponents *components = [[NSDateComponents alloc] init];

[components setDay:6]; 
[components setMonth:5]; 
[components setYear:2004];

NSInteger weekday = [components weekday]; // Undefined (== NSUndefinedDateComponent)

要将一个日期对象解析到相应的date components，你可以使用NSCalendar的components:fromDate:方法。此外日期本身，你需要指定NSDateComponents对象返回组件。 
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NSDate *today = [NSDate date];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *weekdayComponents = [gregorian components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:today];

NSInteger day = [weekdayComponents day]; 
NSInteger weekday = [weekdayComponents weekday];

同样你也可以从NSDateComponents对象来创建NSDate对象： 
NSDateComponents *components = [[NSDateComponents alloc] init];

[components setWeekday:2]; // Monday 
[components setWeekdayOrdinal:1]; // The first Monday in the month 
[components setMonth:5]; // May 
[components setYear:2008];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDate *date = [gregorian dateFromComponents:components];

为了保证正确的行为，您必须确保使用的组件在日历上是有意义的。指定“出界”日历组件，如一个-6或2月30日在公历中的日期值产生未定义的行为。 
你也可以创建一个不带年份的NSDate对象，这样的操作系统会自动生成一个年份，但在后面的代码中不会使用其自动生成的年份。 
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NSDateComponents *components = [[NSDateComponents alloc] init];

[components setMonth:11]; 
[components setDay:7];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDate *birthday = [gregorian dateFromComponents:components];

下面的示例显示了如何从一个日历置换到另一个日历： 
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NSDateComponents *comps = [[NSDateComponents alloc] init];

[comps setDay:6]; 
[comps setMonth:5]; 
[comps setYear:2004];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDate *date = [gregorian dateFromComponents:comps];

[comps release]; 
[gregorian release];

NSCalendar *hebrew = [[NSCalendar alloc] initWithCalendarIdentifier:NSHebrewCalendar];

NSUInteger unitFlags = NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit;

NSDateComponents *components = [hebrew components:unitFlags fromDate:date];

NSInteger day = [components day]; // 15 
NSInteger month = [components month]; // 9 
NSInteger year = [components year]; // 5764

历法计算 
在当前时间加上一个半小时： 
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NSDate *today = [[NSDate alloc] init];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *offsetComponents = [[NSDateComponents alloc] init];

[offsetComponents setHour:1]; 
[offsetComponents setMinute:30];

// Calculate when, according to Tom Lehrer, World War III will end 
NSDate *endOfWorldWar3 = [gregorian dateByAddingComponents:offsetComponents toDate:today options:0];

获得当前星期中的星期天（使用格里高利历）： 
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NSDate *today = [[NSDate alloc] init];

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

// Get the weekday component of the current date 
NSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit fromDate:today];

/* 
Create a date components to represent the number of days to subtract from the current date.

The weekday value for Sunday in the Gregorian calendar is 1, so subtract 1 from the number of days to subtract from the date in question. (If today is Sunday, subtract 0 days.) 
*/

NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];

[componentsToSubtract setDay: 0 - ([weekdayComponents weekday] - 1)];

NSDate *beginningOfWeek = [gregorian dateByAddingComponents:componentsToSubtract toDate:today options:0];

/* 
Optional step: 
beginningOfWeek now has the same hour, minute, and second as the original date (today).

To normalize to midnight, extract the year, month, and day components and create a new date from those components. 
*/

NSDateComponents *components = [gregorian components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate: beginningOfWeek];

beginningOfWeek = [gregorian dateFromComponents:components]; 
如何可以计算出一周的第一天（根据系统的日历设置）： 
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NSDate *today = [[NSDate alloc] init];

NSDate *beginningOfWeek = nil;

BOOL ok = [gregorian rangeOfUnit:NSWeekCalendarUnit startDate:&beginningOfWeek interval:NULL forDate: today];

获得两个日期之间的间隔: 
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NSDate *startDate = ...; 
NSDate *endDate = ...;

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSUInteger unitFlags = NSMonthCalendarUnit | NSDayCalendarUnit;

NSDateComponents *components = [gregorian components:unitFlags fromDate:startDate toDate:endDate options:0];

NSInteger months = [components month]; 
NSInteger days = [components day]; 
使用Category来计算同一时代（AD|BC）两个日期午夜之间的天数： 
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@implementation NSCalendar (MySpecialCalculations)

-(NSInteger)daysWithinEraFromDate:(NSDate *) startDate toDate:(NSDate *) endDate { 
NSInteger startDay=[self ordinalityOfUnit:NSDayCalendarUnit inUnit: NSEraCalendarUnit forDate:startDate];

NSInteger endDay=[self ordinalityOfUnit:NSDayCalendarUnit inUnit: NSEraCalendarUnit forDate:endDate];

return endDay-startDay; 
}

@end

使用Category来计算不同时代（AD|BC）两个日期的天数： 
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@implementation NSCalendar (MyOtherMethod)

-(NSInteger) daysFromDate:(NSDate *) startDate toDate:(NSDate *) endDate {

NSCalendarUnit units=NSEraCalendarUnit | NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit;

NSDateComponents *comp1=[self components:units fromDate:startDate]; 
NSDateComponents *comp2=[self components:units fromDate endDate];

[comp1 setHour:12]; 
[comp2 setHour:12];

NSDate *date1=[self dateFromComponents: comp1]; 
NSDate *date2=[self dateFromComponents: comp2];

return [[self components:NSDayCalendarUnit fromDate:date1 toDate:date2 options:0] day]; 
}

@end

判断一个日期是否在当前一周内（使用格里高利历）： 
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-(BOOL)isDateThisWeek:(NSDate *)date {

NSDate *start; 
NSTimeInterval extends;

NSCalendar *cal=[NSCalendar autoupdatingCurrentCalendar]; 
NSDate *today=[NSDate date];

BOOL success= [cal rangeOfUnit:NSWeekCalendarUnit startDate:&start interval: &extends forDate:today];

if(!success) 
return NO;

NSTimeInterval dateInSecs = [date timeIntervalSinceReferenceDate]; 
NSTimeInterval dayStartInSecs= [start timeIntervalSinceReferenceDate];

if(dateInSecs > dayStartInSecs && dateInSecs < (dayStartInSecs+extends)){ 
return YES; 
} 
else { 
return NO; 
} 
}