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The Salecker-Wigner-Peres (SWP) clock is often used to determine the durationa quantum particle is supposed to spend is a specified region of space $\Om$.By construction, the result is a real positive number, and the method seems toavoid the difficulty of introducing complex time parameters, which arises inthe Feynman paths approach. However, it tells little about about the particle'smotion. We investigate this matter further, and show that the SWP clock, likeany other Larmor clock, correlates the rotation of its angular momentum withthe durations, $\tau$, which the Feynman paths spend in $\Om$, therebydestroying interference between different durations. An inaccurate weaklycoupled clock leaves the interference almost intact, and the need to resolvethe resulting "which way?" problem is one of the main difficulties at thecentre of the "tunnelling time" controversy. In the absence of a probabilitydistribution for the values of $\tau$, the SWP results are expressed in termsof moduli of the "complex times", given by the weighted sums of thecorresponding probability amplitudes. It is shown that over-interpretation ofthese results, by treating the SWP times as physical time intervals, leads toparadoxes and should be avoided. We also analyse various settings of the SWPclock, different calibration procedures, and the relation between the SWPresults and the quantum dwell time. The cases of stationary tunnelling andtunnel ionisation are considered in some detail. Although our detailed analysisaddresses only one particular definition of the duration of a tunnellingprocess, it also points towards the impossibility of uniting various timeparameters, which may occur in quantum theory, within the concept of a single"tunnelling time".

The Salecker-Wigner-Peres clock, Feynman paths, and a tunnelling time that should not exist