Tiene cita con el dentista dentro de una semana.

Tienes PREVISTA una cita con el dentista en una semana. previsto/a is something that you expect to happen but it is uncertain Saludos :)

Hola Stefan. Debes de conseguir una cita con el dentista en una semana. Saludos. Antonio

Tenés que ir al dentista antes de que pase la semana.

I do not quite understand the difference, but with the example of the delivery, I think that I grasp something; you can say something like Le toca visitar al dentista dentro de una semana. This phrase could be in a letter from the dentist office to remember the patient that some time ago an agreement on an appoinment was settled. The letter tries to remember the patient that it's about time to call for the appointment.