The modeling of a set of impedance values given either experimentally or heuristically for implementation as a time-domain impedance-equivalent boundary condition (TDIBC) is discussed. It is shown that impedance as defined by the reflection of plane harmonic waves when extended for broadband time-domain applications can correspond to mathematically feasible but physically unacceptable noncausal reflection processes. Given a set of impedance data on a finite, positive, and real frequency range, it is possible to construct causal time-domain models that render efficient implementation of TDIBC for broadband reflections. However, these models defined by measurement of harmonic waves may not be valid for the reflection of impulses, for which measurement of transient reflections is warranted.