在iOS中，表示时间的类是NSDate，但是NSDate仅仅保存了一个时刻（据1970年1月1日凌辰开始的秒数）。
想获取这个时刻的年，月，日，时，分，秒等等信息，光从NSDate是不行的。
而获取这些元素的途径是通过NSCalendar类来进行的。
下面的代码展示了如何从一个NSDate获取这些元素，并通过指定的元素来创建NSDate对象
//首先创建NSCalendar的实例，可以简单的用当前实例，也可以创建其它的历法对应的实例。
NSCalendar *cal = [NSCalendar currentCalendar];
//下面通过NSCalendar来获取各个元素，保存在类NSDateComponents的实例中
//需要通过函数参数components指定希望获取的元素，详细的枚举后面会列出
NSDateComponents *dateComps = [cal components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit|NSHourCalendarUnit|NSMinuteCalendarUnit fromDate:startDate];
int year = [dateComps year];
int month = [dateComps month];
int day = [dateComps day];
int hour = [dateComps hour];
int minute = [dateComps minute];
int second = [dateComps second];
//也可以通过NSCanlendar和NSDateComponents来创建NSDate
NSDate *newDate = [cal dateFromComponents:dateComps];
enum {
NSEraCalendarUnit = kCFCalendarUnitEra,
NSYearCalendarUnit = kCFCalendarUnitYear,
NSMonthCalendarUnit = kCFCalendarUnitMonth,
NSDayCalendarUnit = kCFCalendarUnitDay,
NSHourCalendarUnit = kCFCalendarUnitHour,
NSMinuteCalendarUnit = kCFCalendarUnitMinute,
NSSecondCalendarUnit = kCFCalendarUnitSecond,
NSWeekCalendarUnit = kCFCalendarUnitWeek /* NS_DEPRECATED(10_4, 10_7, 2_0, 5_0) */,
NSWeekdayCalendarUnit = kCFCalendarUnitWeekday,
NSWeekdayOrdinalCalendarUnit = kCFCalendarUnitWeekdayOrdinal,
#if MAC_OS_X_VERSION_10_6 <= MAC_OS_X_VERSION_MAX_ALLOWED || __IPHONE_4_0 <= __IPHONE_OS_VERSION_MAX_ALLOWED
NSQuarterCalendarUnit = kCFCalendarUnitQuarter,
#endif
#if MAC_OS_X_VERSION_10_7 <= MAC_OS_X_VERSION_MAX_ALLOWED || __IPHONE_5_0 <= __IPHONE_OS_VERSION_MAX_ALLOWED
NSWeekOfMonthCalendarUnit = kCFCalendarUnitWeekOfMonth,
NSWeekOfYearCalendarUnit = kCFCalendarUnitWeekOfYear,
NSYearForWeekOfYearCalendarUnit = kCFCalendarUnitYearForWeekOfYear,
#endif
#if MAC_OS_X_VERSION_10_7 <= MAC_OS_X_VERSION_MAX_ALLOWED || __IPHONE_4_0 <= __IPHONE_OS_VERSION_MAX_ALLOWED
NSCalendarCalendarUnit = (1 << 20),
NSTimeZoneCalendarUnit = (1 << 21),
#endif
};
typedef NSUInteger NSCalendarUnit;
