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Advancements in the Industrial Internet of Things (IIoT) and cyber-physical systems have driven the development of Industry 4.0, which has led to the emergence of advanced cloud-based applications. The web applications connect the field IIoT devices to 'the Internet'. The adequate performance of web applications depends on how seamlessly devices communicate and synchronize. The IIoT devices prominently use Message Queueing Telemetry Transport (MQTT) for machine to machine (M2M) communication. MQTT implements publish-subscribe-based broker architecture that distributes a common timebase to client devices. Such one-way time transfer methods are less accurate and often introduce synchronization errors. While existing time synchronization methods for field IIoT end devices, e.g., Simple Network Time Protocol (SNTP), provide adequate synchronization based on two-way delay transfer methods, their performance degrades significantly with deteriorating network conditions. To address these synchronization challenges, we propose SMQTT, a scalable, software-based, lightweight clock synchronization method for IIoT end devices utilizing MQTT-based M2M communication. The proposed method effectively leverages the broker architecture to implement a two-way delay-based synchronization algorithm. We conducted measurements in network deployments to evaluate the performance of the proposed algorithm. The results demonstrate that SMQTT surpasses both SNTP and state-of-the-art methods in terms of accuracy. Specifically, in our measurements under poor network conditions, SMQTT achieves synchronization performance improvements of 94% and 73% compared to SNTP and the state-of-the-art method, respectively.