For that, don't use the Advertisements function. Use Notices instead.
You can use a script like this for the message:
HTML:
<center>
<a href="http://{URL for the target location if the ad is clicked}">
<img src="https://{URL for the banner image source location}" title="" />
</a>
</center>
Then you can set start and end date/time under page criteria:
View attachment 225472