Path<Date> accountCreatedTime = root.<Date> get("AccountCreatedTime");
Path<Date> firstPostCreatedTime = root.<Date> get("FirstPostCreatedTime");

final Predicate timeInHourPredicate = criteriaBuilder
    .greaterThanOrEqualTo(accountCreatedTime, FirstPostCreatedTime);

select u
from users u
join posts p on p.user_id = u.id
where datediff('hour', u.account_created_time, p.first_post_created_time)