Calculating standardized incidence (or mortality) ratios or performing Poisson regression requires counting the person-years (p-y) generated during the follow-up of a cohort. Several software programs can perform this task, but SAS users must always program this step of the analysis themselves. Various algorithms have been proposed in the literature, but none of them allow for exact calculations. We propose here a simple program that accurately counts person-years and also estimates SMR. It provides a reference tool for performing this step in cohort analysis, using SAS or another language (the basic algorithm can be easily adapted). (R.A.)