When course content is copied from one quarter to another the announcements get copied too. During the course copy Canvas compares the previous course start and end dates to the new course start and end dates and adjust all date settings associated with course events and assignments accordingly, including the 'post at' date for Announcements. This means, new 'post at' dates will be applied to all course announcements that reflect the new course start and end dates. If you do not edit these dates or remove the announcement they will automatically get sent to course users on the mathematically determined date. Due to the individual nature of course announcements you will want to pay special attention to these 'post at' dates and update as needed prior to your course start date.
To prevent the incorrect announcement being sent at the wrong time: