NSDateFormatter returns null for specific dates

I'm having a problem with the NSDateFormatter. It is returning null for some specific dates.

I did this function to debug the case:

- (void) debugTest:(NSString *)dateStr{

    NSLog(@"String: %@", dateStr);

    NSDateFormatter *df = [[NSDateFormatter alloc] init];
    [df setDateFormat:@"yyyy-MM-dd"];

    NSDate *dateObj = [df dateFromString: dateStr];

    NSLog(@"Date: %@", dateObj);

}

This is my log output for some dates:

String: 2012-10-18
Date: 2012-10-18 03:00:00 +0000
String: 2012-10-19
Date: 2012-10-19 03:00:00 +0000
String: 2012-10-20
Date: 2012-10-20 03:00:00 +0000
String: 2012-10-21
Date: (null)
String: 2012-10-22
Date: 2012-10-22 02:00:00 +0000
String: 2012-10-23
Date: 2012-10-23 02:00:00 +0000

Why it is returning null only for the date 2012-10-21?

I did other tests with the interval from 2011-10-01 to 2014-11-01 and the following dates returned null:

2011-10-16, 2012-10-21, 2013-10-20, 2014-10-19

Is this some kind of bug? I am doing something wrong?

I will answer my own question just to register in case of someone get the same problem:

I added to my date formatter [df setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]]; AND [df setTimeZone:[NSTimeZone timeZoneWithName:@"GMT"]];.

So, my new code is:

- (void) debugTest:(NSString *)dateStr{

    NSLog(@"String: %@", dateStr);

    NSDateFormatter *df = [[NSDateFormatter alloc] init];
    [df setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"]];
    [df setTimeZone:[NSTimeZone timeZoneWithName:@"GMT"]];
    [df setDateFormat:@"yyyy-MM-dd"];

    NSDate *dateObj = [df dateFromString: dateStr];

    NSLog(@"Date: %@", dateObj);

}

I think the problem was related to the device's configuration, setting the two properties fixed the problem.