  The large-scale clock network is the key ingredient to obtain high precision
in many scenarios, from fundamental research to cutting-edge applications.
However, time synchronization among microwave clocks has become an upper limit
of the time-frequency network because of the inherent constrained precision in
optical pulse measurements. Here, we demonstrate a femtosecond-level time
synchronization of microwave clocks through a commercial link of 205.86 km via
dual-comb-enhanced optical two-way time transfer, which achieves a 6.23-fs
residual time deviation between synchronized timescales at 1 s and an
instability below 6E-18 at 10,000 s. Further, the high-precision time
synchronization of microwave clocks significantly enhances the probe ability of
subtle reciprocity changes of fiber to the sub-picosecond level. This work
provides a path toward secure fiber time-frequency networks to support future
microwave-clock-based precise timing and sensing systems.
