JPA Criteria API: Difference between two dates in database?

Here is the Explanation of Equivalent JPA Criteria Query of

select * from mytable where until > from + interval 4 week;

First you have to create unit expression and extend it from BasicFunctionExpression for which take "WEEK" parameter as a unit and override its rendor(RenderingContext renderingContext) method only.

import java.io.Serializable;
import org.hibernate.query.criteria.internal.CriteriaBuilderImpl;
import org.hibernate.query.criteria.internal.compile.RenderingContext;
import org.hibernate.query.criteria.internal.expression.function.BasicFunctionExpression;

public class UnitExpression extends BasicFunctionExpression<String> implements Serializable {

  public UnitExpression(CriteriaBuilderImpl criteriaBuilder, Class<String> javaType,
      String functionName) {
    super(criteriaBuilder, javaType, functionName);
  }

  @Override
  public String render(RenderingContext renderingContext) {
    return getFunctionName();
  }
}

then you use this unit expression in your JPA criteria Query.

 CriteriaBuilder cb = session.getCriteriaBuilder();
    CriteriaQuery<MyTable> cq = cb.createQuery(MyTable.class);
    Root<MyTable> root = cq.from(MyTable.class);

    Expression<String> week= new UnitExpression(null, String.class, "WEEK");
    Expression<Integer> timeDiff = cb.function(
        "TIMESTAMPDIFF",
        Integer.class,
        week,
        root.<Timestamp>get(MyTable_.until),
        root.<Timestamp>get(MyTable_.from));
    List<Predicate> conditions = new ArrayList<>();
    conditions.add(cb.greaterThan(timeDiff, 4));
    cq.where(conditions.toArray(new Predicate[]{}));
    return session.createQuery(cq);

It is working fine.

EDIT2: workaround found

Since we have to work only with functions that don't need a built-in parameter (like WEEK), I ended up with

cb.greaterThan(
  cb.diff(
    cb.function("unix_timestamp", Long.class, root.get(Table_.until)),
    cb.function("unix_timestamp", Long.class, root.get(Table_.from))
  )
, 3600L*longerThanHours)

For reference a version that leads to a dead end:

There is no way to make it work like this, you can not send "hour" without surrounding "" as a parameter to the database.

CriteriaBuilder cb = ...;
CriteriaQuery<MyTable> cq = cb.createQuery(MyTable.class);
Root<MyTable> mytable = cq.from(MyTable.class);

cb.greaterThan(
   cb.function(
      "timestampdiff"
      , Integer.class
      , WEEK // <-- this is where JPA gets unable to create SQL
      , mytable.get(MyTable_.from)
      , mytable.get(MyTable_.until)
   )
   , 4
)