I had it set to localtime instead of UTC, which for me is -5 hours. Caused me a bit of grief earlier today after making a new post that started out already five hours old!
It seems like a real pickle though. What if I set my time to a year in the future and make a post? Would that post stay at the top of new all year? I guess lemmy should use the received date rather than what is sent but that could be problematic when things are bogged down or having federation issues.
Thank you everyone! Amazing to see this working with all the other instances. Honestly surprised I didn't have to do too much config after using this post as a rough guide. Just took time for everything to start downloading properly, especially with the chaos of the July 1st exodus.
A new instance doesn't federate on its own. It needs to discover the other servers. Use something like https://lemmyverse.net/ to make links with your home server and visit the other instances. Subscribe to a few and you'll start getting those posts.