Are you sure you really mean "unpair". Pairing is not the same as connecting. Generally, devices are paired once and remain paired indefinitely. Connection is made when you want communication b/w them.
In my projects, connection is initiated from the app (I don't know if it can be initiated from the sketch). The app can also disconnect. I suppose the sketch could request the app to disconnect. It would be up to you to create a code for the sketch to send and be responded to by the app.
To disconnect at a particular time of day, perhaps the simplest way would be to use the phone's time of day clock. The sketch could send a command requesting the app to disconnect at a certain time of day. Might be other strategies to explore.