Several current and future air traffc concepts rely on four-dimensional (4D) trajectory predictions to estimate future states of aircraft in order to better manage traffc flows and de-conflict flights at future points. Interval Management (IM) is a future concept that will rely on 4D trajectories. To conduct an IM operation, an aircraft must be equipped with IM avionics that provide speed guidance to help an aircraft to achieve and maintain a desired spacing interval relative to a Target Aircraft. The avionics predict the relative spacing at downstream points using the Estimated Time of Arrival (ETA) of each aircraft. While many aircraft already have functions to calculate 4D trajectories and ETAs in their Flight Management Systems (FMSs), variability in the performance of the ETA calculations may cause unexpected behavior in the IM speed guidance. This paper compares the perfor- mance of two approaches for calculating ETAs. Preliminary results reveal some diffierences in behavior that may have implications on the design of the IM algorithms that calculate the IM speed guidance.