After creating the promotion via the wizard, customize it or click the Edit button from the list of promotions.
In the second tab, let Discount Ninja know you want this promotion to expire:

Depending on the type of expiration you configure you'll have different options below. In the case of "In x days/hours/minutes..." you have the following options:

You may want the timer to restart every time a visitor opens your shop in a new browser window or tab.

To do this use the option indicated below:

You can display the timer in a pop-up or on the notifications shown on the home page, collection page, product page and cart page. The last notification can be configured separately.
To do this, simply add {{TIMER}} to the text text of the title or message of the pop-up or notification.
Here's an example of a timer in the notification message:

Learn more about how to use Discount Ninja's shortcodes here.