Selecting unique logins in a given time period

My SQL is a little rusty and JPQL is entirely new to me. I have a table with recorded login events, which have a user id and a time. I'm trying to select the unique number of user id's present in a selection of login events that occured in a specified time period:

        Query query = em.createQuery(
                "SELECT COUNT(*) FROM ( " +
                    "SELECT DISTINCT s.userId FROM UserSession s " +
                    "WHERE s.loginTime >= :fromTime " +
                    " AND s.loginTime < :toTime " +
                ") "
        );
        query.setParameter("fromTime", new Date(fromTime));
        query.setParameter("toTime", new Date(toTime));

This give开发者_开发技巧s me a parsing error (the ( after FROM is unexpected) so it's apparently not the way to go. What should I do instead?

Example: If I have the records {A,B,B,A,A,C,A,B,D} all within the date range, I'd like the query to return 4.

Query query = em.createQuery(
                "SELECT COUNT(DISTINCT s.userId) FROM UserSession s " +
                "WHERE s.loginTime >= :fromTime " +
                   " AND s.loginTime < :toTime " +
        );

Source: http://docs.jboss.org/hibernate/entitymanager/3.5/reference/en/html/queryhql.html#queryhql-aggregation