First, to get the Date to show up in the post meta data, go to the Display Tab in the Trellis Settings and scroll down until you see Post Meta Display - Top and Post Meta Display - Bottom. You will use the check boxes in the dropdown menus to select what data you want to display at the top and/or bottom. Make sure you have Date selected for whichever location(s) you want.
Once that's done, we also have a setting in Trellis that allows you to choose either the Published Date, Modified Date, or both to display in your post meta data.
Still, in the Display Tab, you will want to scroll to the setting Post Meta Date Display.
The default is for both the Published and Modified dates to display, but you also have the option to choose either/or.
Display Modified - If you choose this option, the date will change every time you update the post.
Display Published - With this option, only the Published date will display, and will only change if you republish the post.
Display Both - Both the Published date and Modified date will be shown.