The time coordinates of interaction points could be established by extracting the envelop of a transient signal waveform through Hilbert transform and formulating the line equations for the increasing and decreasing segments of an envelope using least square method. The difference of time coordinates lies in the duration time. The duration of increasing and decreasing segments and the whole duration of AC transient signal waveform could be accurately evaluated using waveform measurement and least square method. It was found through analyses and experiments that an uncertainty of less than 0.5ms could be achieved for a SNR of more than 40 dB. It was therefore concluded that the proposed method could be used to measure under-voltage waveform and further extended to do power characteristics testing including compliance test of automotive electronic devices.