m = (extract (month from zdate));
         y = (extract (year from zdate));
         m = m + 1;
         if (m > 12) then
         begin
           m = 1;
           y = y1 + 1;
         end
         date2 = cast('1.'||m||'.'||y as date) - 1;

<количество дней в месяце даты YourDate> =
==========================================
EXTRACT(DAY FROM
CAST('01.'||EXTRACT(MONTH FROM TmpDate)||'.'||
EXTRACT(YEAR FROM TmpDate) AS DATE) - 1);
где
TmpDate =
CAST('01.'||EXTRACT(MONTH FROM YourDate)||'.'||EXTRACT(YEAR FROM YourDate) AS DATE) + 32;