Atrial fibrillation is the most common sustained cardiac arrhythmia. To determine the activation frequency of the atrial fibrillation the frequency-domain analysis is used mostly, however it is unreliable in several cases. This insufficiency led to the demand of using time-domain analysis. Although these attempts seemed to be promising, there have remained several limitations. In our research we developed an enhanced algorithm for time-domain analysis. We defined two indicators which characterized the signals and the reliability of the method well. As compared to the frequency-domain analysis our algorithm can widen the applicability of time-domain analysis since in several cases-when the results of the frequency-domain analysis are unreliable-it can be applied still reliably. As a complementation of the frequency-domain analysis our algorithm can be an extremely helpful tool in medical practice.