Trouble with insert into.

Hello all. Lets say I have:
Table A (username text, first_name text, last_name text, zip int4)
Table B (username text, signupdate date, zip text)

In table A I have
user1 fred jones 53125
user2 bob smith 52145

In table B I have
user2 06/16/1999 52145

The way these tables were populated, table A and B were written to at the same
time. However, table B, did not exist for the first 6 months of use. So there
are many users who are in table A, that are not in table B.

Now what I am tryign to do, is insert all users that dont exist in table B,
that exist in table A, into table B. With o ne twist. Signup date does not
exist in table A, so I need to insert them all with an arbitrary date. I plan
on putting them all in as 04/15/1999.

Here is the select statement I was able to get to work:
select l.username, l.zip
from tableA l
where not exists (
select ld.username
from tableB ld
where ld.username = l.username
);

Now my difficulty comes in inserting the data that is retrieved into table B.

If anyone can help me out, I would greatly appreciate it.

Thanks,

Darren