CREATE TABLE SUPPLY (
    PERIOD        DATE,
    ...);

CREATE TABLE SUPPLY_OPER (
    DATETIME  TIMESTAMP,
    ...);

CREATE OR ALTER trigger supply_oper_aiu0 for supply_oper
active after insert or update position 0
AS
/* Здесь переменные */
begin
  if (updating) then
  begin
    if ((old.datetime <> new.datetime) or (:sp113_new <> :sp113_old) or
             (old.sp122 <> new.sp122)) then
         begin

           wherestr = 'where ((period=?) and (sp113=?) and (sp122=?) and (cost=?))';

           query = 'update supply set '||:supplyfield||'=? '||:wherestr;          

           execute statement (:query) (-old.quantity, cast(old.datetime as date) /* здесь возникает проблема */, 
                                                                     :sp113_old, old.sp122, old.cost);
  end
end

cast(old.datetime as date)

CREATE OR ALTER trigger journal_aiu0 for journal
active after update or delete position 0
AS
begin
  if (updating) then
  begin

    if ((old.isposting = 1) and (new.isposting = 1)
       and (new.ismark = 0) and
       (old.datereg <> new.datereg)) then

       update supply_oper
       set datetime = cast(cast(new.datereg as date)||' '||cast(new.timereg as time) as timestamp)
       where iddoc = new.iddoc;
   end   
end

cast(cast(new.datereg as date)||' '||cast(new.timereg as time) as timestamp)

...
       update supply_oper
       --set datetime = cast(cast(new.datereg as date)||' '||cast(new.timereg as time) as timestamp)
       set datetime = new.datereg + new.timereg
       where iddoc = new.iddoc;
...

  wherestr = 'where ((period=?) and (sp113=?) and (sp122=?) and (cost=?))';
  query = 'update supply set '||:supplyfield||'=? '||:wherestr;          
  execute statement (:query) (-old.quantity, cast(old.datetime as date), :sp113_old, old.sp122, old.cost);