  Waiting time is an important transport quantity that is complementary to
average current and its fluctuation. So far all the studies of waiting time
distribution (WTD) are limited to steady state transport (either dc or ac). In
this work, we present a theory to calculate WTD for coherent electronic systems
in transient regime. We express the generating function of full counting
statistics using Keldysh non-equilibrium Green's functions formalism. Our
analysis goes beyond the wideband approximation and is suitable for first
principles calculation on realistic systems. Analytic solution has been
obtained for short and long time behaviors of waiting times. At short times,
the WTD shows a linear dependence on the waiting time while in the long time
limit, WTD follows Poisson distribution. We have applied this theory to a
quantum dot connected by two leads and calculated cumulants of transferred
charge as well as WTD in the transient regime. We have demonstrated how to
relate WTD to experimental measured data.
