Methods evaluating propositions about timeseries are provided. A service accepts a proposition about one or more timeseries, and the service monitors those timeseries. A proposition can reference multiple timeseries with different time intervals and different units. When new data is available, the service evaluates the proposition to determine if the proposition is true, false, or neither. The service automatically performs unit conversions and selects relevant, previously observed values before evaluating a proposition when new data is available.