Can I schedule an offer to start or stop on a specific date and time? (Essential Upsell app)

No. Essential Upsell does not publish or unpublish offers on a schedule. You turn an offer on or off yourself.

The offer's countdown timer is different. It controls how long the offer is shown to each shopper, not when the offer is live for all shoppers.

To run an offer for a set period, publish it at the start time and unpublish it at the end time. Set a reminder in your calendar so you do not forget.