Timelapse

We discuss the existence in an arbitrary frame of a finite time for the transformation of an initial quantum state into another e.g. in a decay. This leads to the introduction of a timelapse $\tilde\tau $ in analogy with the lifetime of a particle. An argument based upon the Heisenberg uncertainty principle suggests the value of $\tilde\tau =1/M_0$. Consequences for the exponential decay formula and the modifications that $\tilde\tau $ introduces into the Breit-Wigner mass formula are described.