The built in licensing mechanism will access the Internet. If your app is still in its trial period a non working connection to the Internet will lead to an error display on your app.
Since this always happens when it is not convenient you are offered an instant buy option. When your copy of Tabata Timer is not licensed it will, on startup only, show this screen :
By tapping No you can continue evaluating. When tapping Yes you are guided through the purchase process. Beware, after tapping Yes your trial license is waved.
After purchase this screen will no longer appear.