I have discovered through much poking around that libreoffice base using hsqldb 126.96.36.199, and it doesn't support any kind of date_add or interval or anything.
The problem has LONG, loooooong since been fixed in hsqldb 2.0, and we are on version 2.3.4 now.
Date adding is a critical function for many people, yet you can't do it?
Please upgrade this core module.
@Jonathon : or alternatively, you could have poked around in bugzilla and found that there was already a request for this.
*** This bug has been marked as a duplicate of bug 44854 ***