  The measurement of the arrival time of a particle, such as a lepton, a
photon, or a pion, reaching the detector provides valuable information. A
similar measurement for a hadronic final state, however, is much more
challenging as one has to extract the relevant information from a collection of
particles. In this paper, we explore various possibilities in defining the time
of a jet through the measurable arrival times of the jet constituents. We find
that a definition of jet time based on a transverse momentum weighted sum of
the times of the constituents has the best performance. For prompt jets, the
performance depends on the jet trajectory. For delayed jets, the performance
depends on the trajectory of the jet, the trajectory of the mother particle,
and the location of the displaced vertex. Compared to the next-best-performing
jet time definition, the transverse momentum weighted sum has roughly a factor
of ten times better jet time resolution. We give a detailed discussion of the
relevant effects and characterize the full geometrical dependence of the
performance. These results highlight the critical importance of using a proper
definition of jet time with its corresponding detector-dependent calibration
and the exciting possibility of deepening our understanding of jets in the time
domain.
