  We study the effects of time ordering in photon generation processes such as
spontaneous parametric down-conversion (SPDC) and four wave mixing (SFWM). The
results presented here are used to construct an intuitive picture that allows
us to predict when time ordering effects significantly modify the joint
spectral amplitude (JSA) of the photons generated in SPDC and SFWM. These
effects become important only when the photons being generated lie with the
pump beam that travels through the non-linear material for a significant amount
of time. Thus sources of spectrally separable photons are ideal candidates for
the observation of modifications of the JSA due to time ordering.
