I think I manage to understand how to use it. Anybody knows what's the expected "date" param format for SearchQuery?
http://developer.yahoo.com/mail/docs/user_guide/SearchQuery.html

I use something like a timestamp (as the API returns), for example 1338960306 , but I'm getting the error "1338960306 is not a valid dateTime"

Parameters
{"method":"SearchMessages","params":[{"search":{"fid":"Inbox","date":"1338960306","datemode":"since"},"sortKey":"date","sortOrder":"up"}]}

Result
{"result":null,"error":{"code":"Client.InputInvalid","message":"Error: 1338960306 is not a valid dateTime","detail":null}}


Thanks in advance!

Well, I found the date expected format is like this

"2008-08-26T23:35:16Z"

Now, it works. Hope it helps someone else!