You do not need a trigger for that, simply use a timestamp column with DEFAULT TIMESTAMP or the UTC variant. Each time the according row gets updated, the column will be set automatically, unless you specify a value for it in the SET clause of the UPDATE statement.

That untested code snippet should help...

Some information:

• After update triggers are only fired when the contents of the according row has been changed, i.e. something like "UPDATE MyTable SET MyField = MyField WHERE MyPK = 1;" would not fire an after update trigger (well, unless there's a before update trigger that modifieds the row behind the curtains...).
• After update triggers can fire recursively so one needs to prevent that. Here, the trigger is only run when the contents of the according column (here named "dateChanged") has not been changed by the UPDATE statement itself. - When the trigger has run once for the according row, it is called again but then the value is different and the WHEN condition is not fulfilled anymore.
• I generally prefer row-level triggers as they are (IMHO) way easier to code than statement-level triggers.
• As Breck has pointed out, my first attempt to modify the column record-wise within the after update trigger fails with SQLCODE -191. So you have to issue a "real" UPDATE statement there.

create trigger tr_update_dateChanged
after update
   on MyTable
   referencing old as old_row
      new as new_row
for each row 
when (old_row.dateChanged is not distinct from new_row.dateChanged)
begin
   update MyTable set dateChanged = current timestamp
   where MyPk = new_row.MyPk;
end;

This may be what you're looking for; it's a trigger that fires once for each UPDATE statement, and has to deal with old_row and new_row as (possibly multi-row) temporary tables rather than single-row structures, and a FOR cursor loop is used to prevent runaway recursion.

Note that the WHEN clause can't be used for a statement-level trigger, but it can be used in the FOR WHERE clause.

( Funny aside... until today I had no idea IS DISTINCT FROM existed, so thank you Volker for pointing that out :)

CREATE TABLE MyTable ( 
   pkey         INTEGER NOT NULL PRIMARY KEY,
   data         INTEGER NOT NULL DEFAULT 0,
   dateChanged  TIMESTAMP NOT NULL DEFAULT '1900-01-01' );

create trigger tr_update_dateChanged
after update
   on MyTable
   referencing old as old_row
      new as new_row
for each statement 
begin
   FOR f_fetch AS c_fetch INSENSITIVE CURSOR FOR
   SELECT MyTable.pkey AS @pkey
     FROM MyTable
             INNER JOIN new_row
                     ON new_row.pkey = MyTable.pkey
    WHERE MyTable.dateChanged IS NOT DISTINCT FROM new_row.dateChanged
   FOR READ ONLY
   DO
      UPDATE MyTable
         SET MyTable.dateChanged = current timestamp
       WHERE MyTable.pkey = @pkey;
   END FOR;
end;

INSERT MyTable ( pkey, data ) VALUES ( 1, 1 );
INSERT MyTable ( pkey, data ) VALUES ( 2, 1 );
COMMIT;

-- UPDATE with same MyTable.data values.
UPDATE MyTable SET data = 1; -- the trigger should NOT change dateChanged  
COMMIT;
SELECT 'first UPDATE', * FROM MyTable;

-- UPDATE with different MyTable.data values.
UPDATE MyTable SET data = 2; -- the trigger SHOULD change dateChanged  
COMMIT;
SELECT 'second UPDATE', * FROM MyTable;

-- UPDATE with different MyTable.dateChanged values.
UPDATE MyTable SET dateChanged = '2000-01-01'; -- the trigger should NOT change dateChanged   
COMMIT;
SELECT 'third UPDATE', * FROM MyTable;

1 row(s) inserted
1 row(s) inserted
2 row(s) updated
'first UPDATE'        pkey        data dateChanged             
-------------- ----------- ----------- ----------------------- 
first UPDATE             1           1 1900-01-01 00:00:00.0   
first UPDATE             2           1 1900-01-01 00:00:00.0   
(2 rows)

2 row(s) updated
'second UPDATE'        pkey        data dateChanged             
--------------- ----------- ----------- ----------------------- 
second UPDATE             1           2 2019-03-18 10:07:06.519 
second UPDATE             2           2 2019-03-18 10:07:06.52  
(2 rows)

2 row(s) updated
'third UPDATE'        pkey        data dateChanged             
-------------- ----------- ----------- ----------------------- 
third UPDATE             1           2 2019-03-18 10:07:06.524 
third UPDATE             2           2 2019-03-18 10:07:06.524 
(2 rows)