SQL & PL/SQL

i have a problem as follows:-

i have created a table as follows & put primary key constraint as follows:

create table mob_check (no number , dat date)

alter table mob_check add primary key(no,dat)

now, if i put values like -

insert into mob_check values (1,sysdate)
insert into mob_check values (1,sysdate),

it does not consider sysdate as a constraint if there is even some difference in time.
So, please, tell me how to restrict entry of same date, how to put primary key constraint for such a constraint.

regards.