1: NSDate objects
represent a single point in time.
2: NSDateComponents
NSDateComponents encapsulates the components of a date in an extendable, object-oriented manner.
3: NSCalendar
Calendars encapsulate information about systems of reckoning time in which the beginning, length, and divisions of a year are defined. They provide
information about the calendar and support for calendrical computations such as determining the range of a given calendrical unit and adding units to a given absolute time.
In a calendar, day, week, weekday, month, and year numbers are generally 1-based, but there may be calendar-specific exceptions.
To do calendar arithmetic, you use NSDate objects in conjunction with a calendar. For example, to convert between a decomposed date in one calendar
and another calendar, you must first convert the decomposed elements into a date using the first calendar, then decompose it using the second. NSDate provides the absolute scale and epoch (reference point) for dates and times, which can then be rendered into
a particular calendar, for calendrical computations or user display.
转换方法:
NSCalendar 类的
- (NSDateComponents *)components:(NSUInteger)unitFlags fromDate:(NSDate *)startingDate toDate:(NSDate *)resultDate options:(NSUInteger)opts
获取startDate与endDate 之间相隔几天
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDate *startDate = ...;
NSDate *endDate = ...;
unsigned int unitFlags = NSDayCalendarUnit;
NSDateComponents *comps = [gregorian components:unitFlags fromDate:startDate toDate:endDate options:0];
int days = [comps day];
其他使用实例
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];
NSDateComponents *weekdayComponents =
[gregorian components:NSWeekdayCalendarUnit fromDate:date];
int weekday = [weekdayComponents weekday];
提示:
1: NSDate
To parse strings containing dates and to generate string representations of a date, you should use an instance of NSDateFormatter using the methods
dateFromString: and stringFromDate: respectively—see Date Formatters for more details.
转帖：http://o0o0o0o.iteye.com/blog/837634
可以这样做：
NSDate *dateFromString = [dateFormatter dateFromString:@"2012-05-10"];
NSDate *dateToString = [dateFormatter dateFromString:@"2012-07-12"];
int timediff = [dateToString timeIntervalSince1970]-[dateFromString timeIntervalSince1970];
计算结果的单位为秒，自己换算为天就行了。