iphone日期时间指南

主要有3个类：

NSDate：表示一个绝对的时间点

NSCalendar：允许你展示一个特殊的日历，比如Gregorian（公历）或Hebrew历（包括中国农历），它提供了大部分的日期计算接口，并且允许你在NSDate和NSDateComponents之间转换

NSDateComponents允许你展示一个特殊日期的不同部分，比如hour，minute，day，year等等。

NSTimeZone表示时区信息

NSDateFormatter用来在日期和字符串之间转换

可以认为NSDate表示的是绝对的数值，而NSCalendar，NSTimeZone和NSDateFormatter则表示了怎么来解释这个数值。

NSDate：

NSDate是一个抽象的概念，独立于locals，timezones和calendars，也可以认为它表示的就是公历的GMT时间 （NSLog打印的时候都会显示GMT时间）。它的单位是NSTimeInterval，用秒来表示。NSDate用相对于2001年1月1日（GMT） 的秒来计算时间，早于这个时间用负值来表示，晚于这个时间为正值。NSDate的方法timeIntervalSinceReferenceDate就是 指相对于这个日期的秒数。

Listing 1 Creating dates with time intervals

NSTimeInterval secondsPerDay = 24 * 60 * 60;

NSDate *tomorrow = [[NSDate alloc]

            initWithTimeIntervalSinceNow:secondsPerDay];

NSDate *yesterday = [[NSDate alloc]

            initWithTimeIntervalSinceNow:-secondsPerDay];

[tomorrow release];

[yesterday release];

Listing 2 Creating dates by adding a time interval

NSTimeInterval secondsPerDay = 24 * 60 * 60;

NSDate *today = [[NSDate alloc] init];

NSDate *tomorrow, *yesterday;

tomorrow = [today dateByAddingTimeInterval: secondsPerDay];

yesterday = [today dateByAddingTimeInterval: -secondsPerDay];

[today release];

比较时间用isEqualToDate:, compare:, laterDate:和 earlierDate:方法，两个时间之间的间隔用timeIntervalSince...方法

NSCalendar和NSDateComponents：

Listing 3 Creating calendar objects

NSCalendar *currentCalendar = [NSCalendar currentCalendar];

NSCalendar *japaneseCalendar = [[NSCalendar alloc]

                                initWithCalendarIdentifier:NSJapaneseCalendar];

NSCalendar *usersCalendar =

                      [[NSLocale currentLocale] objectForKey:NSLocaleCalendar];

currentCalendar和usersCalendar都是用户本地的日历

Listing 4 Creating a date components object

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

[components setDay:6];

[components setMonth:5];

[components setYear:2004];

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

在Dates和Date Components之间转换：

Listing 5 Getting a date’s components

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];

你需要用Calendar Units这个常量来指定要获取的部分。

NSCalendar的ordinalityOfUnit:inUnit:forDate:方法用来获取一个小的单位在一个大的单位里面的序数，比如你可以查看一个日期在一年中的天数。

Listing 6 Creating a date from components

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];

技术上来说不存在没有年的日期，如果你只想表示生日也可以不指定year：

Listing 7 Creating a yearless date

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

[components setMonth:11];

[components setDay:7];

NSCalendar *gregorian = [[NSCalendar alloc]

                         initWithCalendarIdentifier:NSGregorianCalendar];

NSDate *birthday = [gregorian dateFromComponents:components];

不过这样以后你取components的时候就要记得不能指定year。

Listing 8 Converting date components from one calendar to another

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

日期计算：

你可以用dateByAddingComponents:toDate:options:方法给一个date增加一个date的部分（比如小时或者分钟）

Listing 9 An hour and a half from now

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];

Listing 10 Getting the Sunday in the current week

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];

NSCalendar的rangeOfUnit:startDate:interval:forDate:方法可以按照rangeOfUnit指定的单位查找初始值，比如：

Listing 11 Getting the beginning of the week

NSDate *today = [[NSDate alloc] init];

NSDate *beginningOfWeek = nil;

BOOL ok = [gregorian rangeOfUnit:NSWeekCalendarUnit startDate:&beginningOfWeek

                     interval:NULL forDate: today];

Listing 12 Getting the difference between two dates

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];

Listing 15 Determining whether a date is this week

-(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;

     }

}

Time Zones：

time zone会影响从calendar计算的date components，你可以设置NSCalendar的time zone。NSCalendar默认使用应用程序的时区。可以从timeZoneWithName:, timeZoneWithAbbreviation:, 和 timeZoneForSecondsFromGMT:方法获取一个NSTimeZone对象。可以用[NSTimeZone knownTimeZoneNames]方法获取所有time zone的数组。

可以用setDefaultTimeZone:方法设置默认的时区，用localTimeZone方法始终返回默认的时区。

Listing 16 Creating a date from components using a specific time zone

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

[gregorian setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"CDT"]];

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

[timeZoneComps setHour:16];

//specify whatever day, month, and year is appropriate

NSDate *date=[gregorian dateFromComponents:timeZoneComps];

Date Formatters

有两个基本的方法：dateFromString: 和 stringFromDate:用来在日期和字符串之间转换

 

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];

[formatter setTimeZone:[NSTimeZone systemTimeZone]];

[formatter setDateFormat:@"yyyy-MM-dd"];

NSDate *date = [formatter dateFromString:string];

[formatter release];

 

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];

[formatter setTimeZone:[NSTimeZone systemTimeZone]];

[formatter setDateFormat:@"yyyy-MM-dd"];

NSString *dateString = [formatter stringFromDate:date];

[formatter release];

 

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