iphone日期时间指南
2011-07-21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];
我写的NSDate的几个方法在：https://github.com/phaibin/MyCategories