[en] The relative timing of electron and positron bunches at the SLC interaction point (IP) is affected by phase and energy fluctuations in various upstream systems. By shifting the collision point, IP timing variation may degrade the luminosity and hinder IP spot-size tuning. To monitor the timing stability, in 1997 a novel rf monitor was installed in the South Final Focus, which measures the relative arrival time of the two bunches on every beam pulse. The monitor is based on narrow-band filtering of the beam-induced signal from a common pickup, delay of the first beam waveform into coincidence with the second, and homodyne mixing