BEGIN
if exists(select * from HISDB_OJT2011.seg_billing_encounter as sbe where sbe.encounter_nr = new.encounter_nr and
sbe.bill_dte > str_to_date(concat(date_format(new.date_from, '%Y-%m-%d'), ' ', date_format(new.time_from, '%H:%i:%s')), '%Y-%m-%d %H:%i:%s')) then
call Fail('Cannot insert an accommodation earlier than the latest billing for this encounter!');
end if;
END

So, what exactly is your problem?

Some suggestions.

1) See Information Center or manuals for the syntax of CREATE TRIGGER and general guide.
For example:
IBM DB2 9.7 for Linux, UNIX and Windows Information Center | CREATE TRIGGER statement

2) str_to_date(concat(date_format(new.date_from, '%Y-%m-%d'), ' ', date_format(new.time_from, '%H:%i:%s')), '%Y-%m-%d %H:%i:%s'))
may be replaced by a function
TIMESTAMP(new.date_from , new.time_from)

3) CALl Fail(...) may be replaced by a fnction RAISE_ERROR(...)

wat is date_format equivalent in db2??

u don nid it