Custom types in Postgres, JDBC

Here's a question. Let's say I wanted to create a custom datatype in
Postrgres - something like a timezone2 which would persist the data as
millis. Does anyone have any pointers on where one would start in
thinking about this?

I am pretty sure that I could implement something in our statement
wrapper classes that would allow me to convert timestamps to millis
values and then save them as bigints rather than timestamps. And I
could probably create a couple of stored proc functions on the server
for easily displaying such millis as timestamps, and querying against
them via a timestamp string.

So I guess what I am asking is - how would one actually do it at a
lower level - defining a new type within postgres, and then having
that mapped to a new custom type (which my wrapper classes could then
convert to java.sql.Timestamps). I realize this is more than just a
JDBC question, but JDBC certainly plays a role in it.

Any thoughts on whether this is a good idea or a bad one? Any pointers? Caveats?

Thanks,
Christian