Tracked per customer. The countdown starts from when that specific customer first visited the page. If they return an hour later, the timer continues from where it left off.
Set a fixed expiry date. Everyone sees the same countdown to that date. For example, set it to expire on November 20th and all visitors see "5 days remaining."
Set the timer background colour. Dark colours (red, orange) display white text. Light colours (yellow) display darker text. Red works well for urgency.