A new timing detection method based on acousto-optic modulation is demonstrated. The timing detector is immune to dispersion effects and the environmental and laser amplitude noise can be well suppressed by a balanced configuration. With 1 mW power per pulse train, the measured timing noise floor is about 1×10-10 fs2/Hz, which is close to the shot noise limit. The integrated timing jitter is 26 as at [1 Hz, 1 MHz]. With 170 fs pulse width and typical detector parameters, the calculated detector's timing noise floor is more than 5 and 12 orders of magnitude lower than that of a BOC, at 1 mW and 1 µW input power, respectively. This timing detector has a variety of potential applications in ultra-long fiber link stabilization, quantum metrology, weak signal timing control, etc.