How do RSS readers read different feeds?

sorry for a silly question, (I know it is). I didn't work with RSS before and I have a question. I wrote an RSS reader for iphone whi开发者_开发技巧ch can read one particular feed, if I try to read some other feed it doesn't work properly, I have some data missing, or sometimes it even crashes. Ok, it is naturally enough, because different feeds have different date formats, different xml elements names etc. So I don't understand how all this readers where you only need to provide url parse every feed

Thank you very much))

---

Just make sure your app is adhering to RSS Standards

W3C has a validation tool to verify that the RSS you're consuming is good coming in.