Unable to convert date to tm

Hi

I have a column x in a table of type date and i have an index on x
runing the query "SELECT * FROM table WHERE x >= '2004-01-01"
i get this:
Unable to convert date to tm

where using a cast like this: "SELECT  * FROM table WHERE x  
>='2004-01-01::timestamp" result is ok but it doesn't use index.

Thanks in advance.