NSDate，NSCalendar

NSLog(@"%@",[NSDate date]);

*********************** 时间转字符串*************************
1     NSDate *now = [NSDate date];
NSDateFormatter *format = [[NSDateFormatter alloc]init];
format.dateFormat = @"yyyy-MM-dd HH:mm:ss";
NSString *nowString = [format stringFromDate:now];
NSDate *newTime = [NSDate date];
CGFloat delta = [newTime timeIntervalSinceDate:now];

//NSLog(@"%@,%f",nowString,delta);
//打印结果：

********************字符串转时间*******************************
NSString *str = @"2015-03-30 19:33:00";
NSDate *nowDate = [format dateFromString:str];
//NSLog(@"%@",nowDate);

**********************日历******************************************
NSCalendar *calendar = [NSCalendar currentCalendar];
//NSLog(@"%@",calendar);
NSCalendarUnit unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
NSDateComponents *components = [calendar components:unit fromDate:now toDate:nowDate options:0];
NSLog(@"%@,%zd,%zd,%zd",components,components.year,components.month,components.day);
打印结果：

NSInteger year = [calendar component:NSCalendarUnitYear fromDate:nowDate];
NSInteger month = [calendar component:NSCalendarUnitMonth fromDate:nowDate];

NSLog(@"%zd,%zd",year,month);
打印结果：

NSCalendar *calendar = [NSCalendar currentCalendar];
NSString *oldTime = @"2014-12-31 23:59:59";
NSString *newTime = @"2015-01-01 00:00:01";
NSDateFormatter *format = [[NSDateFormatter alloc]init];
format.dateFormat = @"yyyy-MM-dd HH:mm:ss";
NSDate *oldDate = [format dateFromString:oldTime];
NSDate *newDate = [format dateFromString:newTime];

NSCalendarUnit unit = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
NSDateComponents *comp = [calendar components:unit fromDate:oldDate toDate:newDate options:0];

NSLog(@"%@",comp);