Stream-based monitoring is a runtime verification approach where a monitor aggregates streams of input data from sensors and other sources to give real-time statistics and assessments of a system’s health. One of the central challenges in designing reliable stream-based moni- tors is to deal with the asynchronous nature of data streams: in concrete applications, the different sensors being monitored produce values at dif- ferent speeds, and it is the monitor’s responsibility to correctly react to the asynchronous arrival of different streams of values. To ease this process, modern frameworks for stream-based monitoring such as RT- Lola enable users to finely specify data synchronization policies via a system of pacing annotations. While this feature simplifies the design of monitors, it can also lead users to write inconsistent policies, where synchronization between two streams is explicitly requested via annota- tions, but cannot always be achieved. To mitigate this issue, this paper presents pacing types, a novel type system implemented in RTLola to ensure that monitors for asynchronous streams are free of timing incon- sistencies. We give a formal semantics to pacing annotations for a core fragment of RTLola, and present a soundness proof of the pacing type system. For an additional level of guarantees, we machine-checked the soundness proof using the Rocq proof assistant.