Quote Originally Posted by frogbeastegg View Post
There isn't much you can do. The event offering the chance to build a Nanban port triggers from west to east. It moves slowly and I would not be surprised if it had a set date trigger for each clan.

You might be able to convert if you take over a province with a Nanban port already constructed. I doubt this is feasible for an early-game Oda; Kyushu is a long way away.
I don't think there is a date. I've been offered it 3 or 4 times in my Chosokabe campaign now and I'm on turn 20 or something.