Can I show a countdown timer only on products in one category?

Yes. Go to Products → Countdown Timer and create a new timer, or click a timer's title in the Campaigns list to edit it.

In the targeting section at the bottom of the form, choose:

• All products: every product page.
• Specific products: pick the exact products.
• Categories: pick one or more categories, such as Footwear.

Make sure Active is ticked, then click Create (or Update when editing). The timer only appears on product pages that match your choice.