We present a new approach for event detection and accurate determination of the onset time of seismic phases. We use a simple time-domain function (Tpd), which is readily and efficiently calculated in real time. Our function is based on the Tp function, which has been used to characterize the predominant period of a waveform and which was originally introduced by Nakamura (1988) for use in real-time magnitude estimation. We find that while Tp itself produces changes at the onset of a seismic signal, it is not suitable for detecting seismic onsets. However, our modified damped predominant period function, Tpd, has considerable potential for detecting such onsets. We show examples of waveforms with their corresponding Tpd functions and demonstrate how the choice of parameters influences the function. The examples given show clear increases in Tpd for both the P- and S-wave onsets. We implement an approach to determining seismic onsets based on the Tpd function and call it the Tpd picker. The Tpd picker is trained, and we evaluate its performance on four days of continuous data. The Tpd picker compares favorably with a trained short-term average/long-term average picker—there are improvements in the number of accurate picks but, more notably, there is a significant reduction in missed picks. We analyze the signal-to-noise ratios of the waveforms and find that the improvements are mainly, but not exclusively, for cases of high noise. The aforementioned facts, combined with the simplicity and efficiency of the Tpd calculation, implies that there is considerable potential for its use as an automatic picker. The results also suggest that the Tpd picker could be further improved by refining the position of the pick after a seismic phase has been detected.