Finally got the time to write it down, but if you update to the latest version, you can access the help page for both currently available dynamic expressions you can use in the title. It will list any future expressions if there ever are any new ones.

You can access it at /post/help/expressions on your Lemmy Schedule instance (for example, https://schedule.lemmings.world/post/help/expressions). A link for that page has been added to the title field as well.