The paper presents a compact optical frequency division system that uses microresonator-based frequency references and comb generators to achieve zeptosecond-level timing noise in microwave oscillators. The system stabilizes a soliton microcomb formed in an integrated Si$_3$N$_4$ microresonator to two lasers referenced to an ultrahigh-Q MgF$_2$ microresonator. Photodetection of the soliton pulse train produces 25 GHz microwaves with an absolute phase noise of -141 dBc/Hz (547 zs Hz$^{-1/2}$) at a 10 kHz offset frequency. The synthesized microwaves are tested as local oscillators in jammed communication channels, showing improved fidelity compared to those derived from electronic oscillators. This work demonstrates unprecedented coherence in miniature microwave oscillators, providing key building blocks for next-generation timekeeping, navigation, and satellite communication systems. The system's performance is superior to commercial electronic oscillators, with timing noise below 546 zs Hz$^{-1/2}$ at a 10 kHz offset frequency, and it outperforms table-top electronic microwave oscillators in anti-interference experiments, maintaining signal fidelity under heavy jamming conditions.The paper presents a compact optical frequency division system that uses microresonator-based frequency references and comb generators to achieve zeptosecond-level timing noise in microwave oscillators. The system stabilizes a soliton microcomb formed in an integrated Si$_3$N$_4$ microresonator to two lasers referenced to an ultrahigh-Q MgF$_2$ microresonator. Photodetection of the soliton pulse train produces 25 GHz microwaves with an absolute phase noise of -141 dBc/Hz (547 zs Hz$^{-1/2}$) at a 10 kHz offset frequency. The synthesized microwaves are tested as local oscillators in jammed communication channels, showing improved fidelity compared to those derived from electronic oscillators. This work demonstrates unprecedented coherence in miniature microwave oscillators, providing key building blocks for next-generation timekeeping, navigation, and satellite communication systems. The system's performance is superior to commercial electronic oscillators, with timing noise below 546 zs Hz$^{-1/2}$ at a 10 kHz offset frequency, and it outperforms table-top electronic microwave oscillators in anti-interference experiments, maintaining signal fidelity under heavy jamming conditions.