A method of calculating a time difference is disclosed. The method includes sampling a first ultrasonic signal (r21) to produce a first sampled signal (y1(i)) and sampling a second ultrasonic signal (r12) to produce a second sampled signal (y2(i)). A first time (LEAD_LAG) is determined between a time the first sampled signal crosses a threshold (θ1) and a time the second sampled signal crosses the threshold. The first sampled signal is cross correlated with the second sampled signal to produce a second time (SAMP_OFFSET). The time difference is calculated in response to the first and second times.