SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table t ( d datetime);
insert into t values('2019-10-01 00:00:00.000')
1 rows affected

insert into t values('2019-10-01 00:00:00.000')
1 rows affected

update 
t1
set d= d+ cast('17:59:59.000' as datetime)
from t t1
where cast( d as time)='00:00:00.000'
2 rows affected

select * from t


d


01/10/2019 17:59:59

01/10/2019 17:59:59