Performing Date/Time Math In HQL?

Why do you need to do it in the query? Why not just handle it in the java code.

for example:

From RandomThing
Where randomTime is not null and
      randomTime >= :currentTimestamp and
      randomTime <= :maxTimestamp

And then just set the parameters.

You could determine the syntax to do it using SQL in your database and then define a function within a custom HibernateDialect. For example, we needed a weekday function which is not standard SQL. We subclassed the dialect for each database and then added a line like this:

registerFunction("weekday", 
  new SQLFunctionTemplate(Hibernate.INTEGER, "to_char(?1,'D')") );

In your case, you could use a function called date_diff which might be defined as ? - ? in some databases or something different in others. That way you don't have to write raw SQL in your query and if you ever need to switch databases, you just map the function differently in your dialect.

If you need the database server's time, you could first do a simple hql query to get the timestamp and then calculate the maxTimestamp in java and pass the fetched timestamp and the calculated maxTimeStamp to a query like the one of ccclark.

Does it have to be HQL? I would probably switch up to a hibernate criteria and use:

Criteria.add( Restrictions.SQLRestriction( "{alias} <= current_timestamp() " ) )
Criteria.add( Restrictions.SQLRestriction( "{alias} >= (current_timestamp() + ?) ", 5 )