How to Select a Tupl by Nearest Date

Hello

Assume I have a table like
create table foo (
id serial,
date foodate,
primary key(id)
);

with 2 tupls
insert into foo(foodate) values('2008-07-07'); --id = 1
insert into foo(foodate) values('2008-07-04'); -- id = 2

What I need is to select the nearest tupl by a given date and I do not know how to do this.

Something like:
select id from foo where foo date = nearest('2008-07-06');
-> should return 1

select id from foo where foo date = nearest('2008-07-05');
-> should return 2

How can I do this? Note: I have a large Table (> 5'000'000 rows) so a good performing way would be a welcome asset :)

Thanks
Christian
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