The following is taken straight from the Firebird Book, Supplement 2,
Appendix I Function Summary, p. 249:

SQL> SELECT DATEADD(WEEK, 1, DATE 'YESTERDAY') FROM RDB$DATABASE;;

Result (in ISQL, Firebird 2.1, Windows XP):

DATEADD
===========
Statement failed, SQLCODE = -833
expression evaluation not supported

DAY, MONTH, YEAR work as expected. And in the notes for the DATEADD
function WEEK is missing. So I'm not quite sure if it really should work.

Herta