在iOS中，表示時間的類是NSDate，但是NSDate僅僅保存了一個時刻（據1970年1月1日凌辰開始的秒數）。

想獲取這個時刻的年，月，日，時，分，秒等等信息，光從NSDate是不行的。

而獲取這些元素的途徑是通過NSCalendar類來進行的。

下面的代碼展示了如何從一個NSDate獲取這些元素，并通過指定的元素來創建NSDate對象

//首先創建NSCalendar的實例，可以簡單的用當前實例，也可以創建其它的歷法對應的實例。

NSCalendar *cal = [NSCalendar currentCalendar];

//下面通過NSCalendar來獲取各個元素，保存在類NSDateComponents的實例中

//需要通過函數參數components指定希望獲取的元素，詳細的枚舉后面會列出

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];

//也可以通過NSCanlendar和NSDateComponents來創建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;