Yahoo! Contacts API

I'm developing a Yahoo Messenger client for desktop computers (Windows XP) to learn more about Yahoo's APIs, and I've so far successfully written code to get a list of contacts, send messages, get messages, and of course log in and log out. However, now I'm trying to figure out how to add Contacts; I have viewed the documentation for the Yahoo! Contacts API and it is unclear how you are supposed to do authorization, so I figured it was the same as the Messenger API. Whenever I use the same authorization (through the authorization header) for the Contacts API, I get a "timestamp_refused" error. I am using the PLAINTEXT encryption method and I read somewhere in the documentation that when using PLAINTEXT the timestamp was ignored (as such, I am not sending any value for it). I also tried sending timestamp I got from "http://www.unixtimestamp.com/index.php", which yielded the same error (I made sure it was near the correct time).

The following is the request URL I am using:
"http://query.yahooapis.com/v1/yql?q=SELECT%20*%20FROM%20social.contacts%20WHERE%20guid%3Dme&diagnostics=true"

I am setting the Authorization header accordingly, though the timestamp error is given every time. Help?