Both client and server devices must be on the same local network.
The server part defines a list of talk times and the ending hour of the session. The app calculate the advance or delay. The server user sends new countdown times at the beginning of each talk and can also send text message to the speaker. The new countdown time integrate the delay proportionally to end the session on time.
The client part acts like a simple countdown timer and stopwatch. The countdown is remotely controlled. The stopwatch can be started remotely or locally. And text field can receive messages from the server user.