Methods of survival analysis (e.g., the Cox proportional hazards model) require that the event time be measured with respect to some origin time. The choice of origin time is substantively important because it implies that the risk of the event varies as a function of time since that origin. Ideally, the origin time is the same as the time at which observations begin, on the other hand, observations do not begin until some time after the origin time. These late entries are treated as left truncated data in the statistical literature.