If upon registration, the countdown timer is showing 0 and redirecting to an expired page or somewhere else, the reason is mostly due to the settings in the "Page and Link Behavior" under the Advanced tab.

If your on-demand event is scheduled at a high frequency, i.e., every 10, 20, or 30 minutes, then in the field 'On Registration: show registration for an Event +/- from Event starting time:' make sure you set the value to -1.

For any questions, please reach out to Support.