It is surprisingly hard to obtain accurate and precise measurements of the time spent executing a query. We review relevant process and overall measures obtainable from the Linux kernel and introduce a structural causal model relating these measures. A thorough correlational analysis provides strong support for this model. Using this model, we developed a timing protocol, which (1) performs sanity checks to ensure validity of the data, (2) drops some query executions via clearly motivated predicates, (3) drops some entire queries at a cardinality, again via clearly motivated predicates, (4) for those that remain, for each computes a single measured time by a carefully justified formula over the underlying measures of the remaining query executions, and (5) performs post-analysis sanity checks. The resulting query time measurement procedure, termed the Tucson Protocol, applies to proprietary and open-source DBMSes.