with
  SECUENCE as
  (select cast(:DATE_FROM as date) + r0.RDB$RELATION_ID + r1.RDB$RELATION_ID * 30 as ADATE
   from RDB$DATABASE d
   join RDB$RELATIONS r0 on r0.RDB$RELATION_ID < 30
   join RDB$RELATIONS r1 on r1.RDB$RELATION_ID < 30
   order by 1)
select distinct substring(ADATE from 1 for 7)
from SECUENCE
where ADATE <= :DATE_TO