NSDateFormatter dateFromString conversion

NSDateFormatter dateFromString conversion

I am having problems with conversion of NSString object to NSDate. Here is
what I want to do: I am downloading an RSS Message from the internet. I
have the whole message parsed by NSXMLParser. After that, I have parts
like , or saved to particular NSStrings. I want to convert element (that
includes publication date of RSS Message) to NSDate so that I could
perform some operations on it like on a date object (e.g. sorting, showing
on a clock etc.). Here is the way my looks like:
"Wed, 25 Sep 2013 12:56:57 GMT"
I tried to convert it to NSDate in this way:
*//theString is NSString containing my date as a text
NSDate *dateNS = [[NSDate alloc] init];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"EEE, dd MMM yyyy hh:mm:ss ZZZ"];
dateNS = [dateFormatter dateFromString:theString];*
However, after doing above code, dateNS always appear to be (null).
My question is simple: what is the right way to convert NSString with date
formatted like this to NSDate object?
By the way, I have seen the website
http://www.unicode.org/reports/tr35/tr35-25.html#Date_Format_Patterns It
seems that there are many ways to format particular date, but I could not
find what I am doing wrong.