NSDateFormatter 的一个小陷阱:

NSString *ds = @"2013-03-01 23:55:56";
NSDateFormatter* formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"YYYY-MM-dd HH:mm:ss"];
NSDate *date1 = [formatter dateFromString:ds];
NSLog(@"date1: %@", date1); //date1: 2012-03-01 15:55:56 +0000
[formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSDate *date2 = [formatter dateFromString:ds];
NSLog(@"date2: %@", date2); //date2: 2013-03-01 15:55:56 +0000

格式化以后是一年前的一个时间点,yyyy 指代的就是常规意义上的年,而 YYYY 是 Week of Year, 具体解释参考 Wikipedia: ISO_week_date

iOS 开发文档有相关提示:

It uses yyyy to specify the year component. A common mistake is to use YYYY. yyyy specifies the calendar year whereas YYYY specifies the year (of “Week of Year”), used in the ISO year-week calendar. In most cases, yyyy and YYYY yield the same number, however they may be different. Typically you should use the calendar year.