unique index on variable time

hello,

I am interested to know if I can define an unique index on a timestamp
column to reject values within one hour.

insert into table(timestamp_col) values(LOCALTIMESTAMP);
insert into table(timestamp_col) values(LOCALTIMESTAMP + '5
minutes'::INTERVAL);
I want the second insert to fail with "unique violation".

is it possible to do something like that ? I am not interested in
"date_trunc" because I want it to work and for minute 59.

thanks,
Razvan Radu