The timeline is a new block that's been added to the eClass dashboard in the summer 2023 upgrade. It gives students and instructors a heads-up for upcoming due dates and cutoff dates:

By default, it will only show dates for quizzes and assignments, but external tools such as Assign2 aren't able to populate it. This can be worked around by enabling activity completion tracking in your course, and setting an "expect completed on" date in the activity's settings. 

Completion tracking has more advanced features, like restricting course material until another item is completed. We have a more general article about completion tracking in this article.

To enable completion tracking in your course:
Navigate to your course settings and find the "Completion tracking" section, then set "Enable completion tracking" to "Yes":

Activities will now all have an "Activity completion" section in their settings. For this example, we'll enable completion for an Assign2 activity, first by navigation to its settings page, by activating edit mode, then clicking the ellipsis (⋮) icon and selecting 'edit settings'

Enable completion tracking here, the default setting, "students can manually mark the activity as completed" will suffice for most purposes, but for a more detailed description of these settings see this article. You'll then also be able to enable an "expect completed on" date which will show in the dashboard timeline for students and instructors, after the settings are saved.