Under "View", "Master", "Slide Master", your PowerPoint template should have
a "Date/Time" placeholder. Click "View", "Header/Footer" and check to see
that the Date/Time placeholder is set to "Update Automatically', then select
the dropdown for the format you want. You can also drag this placeholder to
the desired location. If by chance your Slide Master does not have this
placeholder, you can right-click a blank part of the slide and select
"Master Layout" then click the placeholder option to insert it.