to_timestamp() is a database function, but you are using JPQL for your query.
For the JPQL timestamp format see,
http://en.wikibooks.org/wiki/Java_Persistence/JPQL#Literals
If you really want to use a database function, you can use the FUNC or FUNCTION operator.
You may also want to declare the constant as a parameter instead, then just set it as a parameter when you execute the query.