Silica microcombs have a high potential for generating tens of gigahertz of optical pulse trains with ultralow timing jitter, which is highly suitable for higher speed and higher bandwidth information systems. So far, the accurate characterization of timing jitter in microcombs has been limited by the measurement methods-although theoretically predicted to be >20dB better performance, the true performance has not been accurately measured until now. Here, using a self-heterodyne based measurement method with 20 zs/Hz1/2 resolution, we show that 2.6-fs rms timing jitter is possible for 22-GHz silica microcombs. We identified their origins, which suggests that silica microcombs may achieve 200-as-level jitter by better intensity noise control. This jitter performance can greatly benefit many high-speed and high-bandwidth applications including analog-to-digital conversion, microwave generation, and optical communications.