NSDate（NSCalendar，NSDateComponents，NSDateFormatter）

－1 NSString 转成 NSDate

    NSString *time = @"2014-05-18 09:24:40";

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

    dateFormatter.dateFormat = @"yyyy-MM-dd HH:mm:ss";

    NSDate *date = [dateFormatter dateFromString:time];

    NSLog(@"%@",date);

－2 获取本地时间 （将格林威治时间转换成本地时间）

    NSDate *date = [NSDate date];

    NSTimeZone *zone = [NSTimeZone systemTimeZone];

    NSInteger interval = [zone secondsFromGMTForDate:date];

    NSDate *localeDate = [date dateByAddingTimeInterval:interval];

    NSLog(@"%@",localeDate);

－3 将 NSDate 转成 NSString

    NSDate *date = [NSDate date];

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

    dateFormatter.dateFormat = @"yyyy-MM-dd HH:mm:ss Z";//"Z"表示时区

    NSString *currentDate = [dateFormatter stringFromDate:date];

    NSLog(@"%@",currentDate);

　　使用  NSDateFormatter 会自动将时间转换成本地时间输出

-4 使用 NSCalendar 和 NSDateComponents 获取时间要素

    NSDate *date = [NSDate date];

    NSCalendar *calendar = [NSCalendar currentCalendar];

    int unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;

    NSDateComponents *dateComponents = [calendar components:unit fromDate:date];

    NSLog(@"现在时间是%li年%li月%li天%li小时%li分钟%li秒", dateComponents.year, dateComponents.month, dateComponents.day, dateComponents.hour, dateComponents.minute, dateComponents.second);

－5 比较两个时间的时间差（这里以格林威治时间和本地时间做示例）

    NSDate *date = [NSDate date];

    NSTimeZone *zone = [NSTimeZone systemTimeZone];

    NSInteger interval = [zone secondsFromGMTForDate:date];

    NSDate *localeDate = [date dateByAddingTimeInterval:interval];

NSCalendar *calendar = [NSCalendar currentCalendar];

    int unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;

    NSDateComponents *dateComponents = [calendar components:unit fromDate:date toDate:localeDate options:0];
NSLog(@"格林威治时间和本地时间相差%ld年%ld月%ld天%ld小时%ld分钟%ld秒", dateComponents.year, dateComponents.month, dateComponents.day, dateComponents.hour, dateComponents.minute, dateComponents.second);

NSDate（NSCalendar，NSDateComponents，NSDateFormatter）