Fix Scenario errors in add_year()

[glatterf42]

Closes #578.
In that issue, @gidden mentioned that this branch by Behnam contained a fix for the issue. However, that branch has become severely outdated over the years. So after rebasing #494, I created a new branch here that captures the changes contained in @behnam-zakeri's branch.

I'm not sure what's still missing in #494, but I think this could be a good opportunity to move things along. And the assessments in #578 made me think that things need to move along to address these known bugs.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[glatterf42]

Ideally, this PR would add a test that confirms that the fix here actually does what we intend it to do.