Feature: Improve date handling of announcements

[AlexCPU]

Thinking about how we're likely to use announcements, I'd expect any PR making visible changes to include a new announcement file explaining the changes, so the users are informed.

We need some way of making that announcement stay visible for 3 days after the PR is merged, rather than the date that the code is committed. Obviously I could push a commit to update the date before merging the changes, but something automatic would be nice.