Free Trial Period
For any subscription, you can set up a free trial period that lets users try your subscription content before buying it. The trial period runs for the period of time that you set and then automatically converts to a full subscription managed according to the subscription's billing interval and price.
To take advantage of a free trial, a user must "purchase" the full subscription through the standard In-app Billing flow, providing a valid form of payment to use for billing and completing the normal purchase transaction. However, the user is not charged any money, since the initial period corresponds to the free trial. Instead, Google Play records a transaction of $0.00 and the subscription is marked as purchased for the duration of the trial period or until cancellation. When the transaction is complete, Google Play notifies users by email that they have purchased a subscription that includes a free trial period and that the initial charge was $0.00.
When the trial period ends, Google Play automatically initiates billing against the credit card that the user provided during the initial purchase, at the amount set for the full subscription, and continuing at the subscription interval. If necessary, the user can cancel the subscription at any time during the trial period. In this case, Google Play marks the subscription as expired immediately, rather than waiting until the end of the trial period. The user has not paid for the trial period and so is not entitled to continued access after cancellation.
You can set up a trial period for a subscription in the Developer Console, without needing to modify or update your APK. Just locate and edit the subscription in your product list, set a valid number of days for the trial (must be 7 days or longer), and publish. You can change the period any time, although note that Google Play does not apply the change to users who have already "purchased" a trial period for the subscription. Only new subscription purchases will use the updated trial period. You can create one free trial period per subscription product.