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;
}
}