That are 2 questions.
1. Program it. I did it 2 weeks ago by adapting (to count from 15 to 0) a script (that counted time) from w3schools.com: https://www.w3schools.com/howto/howto_js_countdown.asp
2. That's a good question for an A/B-test. For a few products, show half of the pages with (A) and the other half (B) without a timer. Then compare the conversion rates of A and B and you'll know.