Methods and apparatus are provided for determining a defibrillation treatment protocol in an external defibrillator using a measurement of elapsed time. The present invention provides a defibrillator with a timer function. Upon activation of the defibrillator, an internal timer begins to run. By closely associating the activation of the defibrillator with the onset of the patient's attack, and by making allowances for inherent time differences between these events, the timer provides a measure of the elapsed time between the onset of the patient's emergency and the presentation of the defibrillator at the patient's side. Using this measure of elapsed time, the defibrillator determines an appropriate treatment therapy, such as CPR or defibrillation therapy.