Is it appropriate to use a timestamp as a DISTKEY in Redshift?
It's pretty corner-case, but there are cases where data location matters slightly.

Let's say you ask the database to do this query:

SELECT COUNT(DISTINCT ip)
FROM test_table
GROUP BY country

If the table is distributed by country, no network activity is needed (I tested this to confirm). For any other distribution style, the hash table will logically need to be re-distributed over the network (I also tested this to confirm).

That said, you probably want to just choose an EVEN distribution style to maximize the scanning speed. For that matter, maybe you want to use Spectrum for this use case.