Hi @GeorgeCollier ...
Unfortunately, announcement end dates aren't a "thing" in Canvas right now. I know a lot of people have wanted this kind of feature for course announcements, so I'd like to point you to a handful of links:
The first link is part of a larger theme (the second link I provided) for course announcements. When taking a look at the second link, under the heading "Referenced Ideas", you'll see a link to the "Start and End Dates" idea (the first link I provided). I'll encourage you to follow these ideas so that if/when the ideas open for voting in the future, you'll be able to participate in that process and any conversations around the idea. See the section on "Ideas and Themes" here: Community - Instructure Community (canvaslms.com).
I know this might not be an exact answer to your question, but I hope it helps in some way.