  We derive the Fermi's golden rule in the Gaussian wave-packet formalism of
quantum field theory, proposed by Ishikawa, Shimomura, and Tobita, for the
particle decay within a finite time interval. We present a systematic procedure
to separate the bulk contribution from those of time boundaries, while
manifestly maintaining the unitarity of the $S$-matrix unlike the proposal by
Stueckelberg in 1951. We also revisit the suggested deviation from the golden
rule and clarify that it indeed corresponds to the boundary contributions,
though their physical significance is yet to be confirmed.
