Modeling concentration data with imprecise sampling time

All: I have a dataset following a constant rate 1-hour infusion where the time of 1-hour sampleis imprecise. In some cases the "1-hr" sample was taken prior to infusion termination as it was intended; however, in others it was quite apparently take after the infusion ended. I'm thinking of modeling with the time of the "1-hr" sample as a "theta". Does anyone have a NONMEM coding example showing how to do this? Any other suggestions as to how to handle this would be appreciated as well. Thanks, Michael