create index idx_zrn_tt1 on
zrn_tt1(dat)
/

insert into zrn_tt1 values ('09/2005');
insert into zrn_tt1 values ('05/2010');
insert into zrn_tt1 values ('01/2015');
insert into zrn_tt1 values ('02/2017');
insert into zrn_tt1 values ('01/2017');
insert into zrn_tt1 values ('10/2007');
insert into zrn_tt1 values ('07/2013');
/

with x as (
select
dat
from zrn_tt1
order by (substr(dat,4)||substr(dat,1,2)) desc
)
select dat
from x where rownum = 1



"very quick and dirty"