If you are just renewing your existing level, they do not send out new credentials until the start of the new program year. Your existing Plat credentials are good until then, so there's no need to send them early and they don't know if you might qualify for Diamond before the end of the year.
This. If you are making a level that you don't currently have this year, they may send out physical credentials (they did for me but that was 5 years ago so YMMV).
If you are re-making a level you already have, assuming it's the same as in years past, you (may) get a new packet sometime early next year. I've always gotten a new one anyhow as a DM, but 2018 may bring "enhancements".