We develop a Green's function formalism and with it we are able to obtain c
lose expressions for the tunnelling and for the reflection times. For each
problem, there appear two characteristic times which correspond to the real
and imaginary parts of the integral of the Green's function at coinciding
coordinates. The time related to the imaginary part represents the minimum
uncertainty of the measurement. A strong analogy between the results of the
different existing approaches is established. and we show that their main
differences are due to finite-size effects.