Elapsed time is calculated as:

1. Time intervals, like displacements, are differences between two readings. 2. Elapsed time is the difference between the ending time and the beginning time. 3. If the starting time is taken to be zero, the elapsed time is simply the final clock reading. 4. Subtracting in the other order would give a negative interval for forward motion. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 2 'Kinematics', sections 2.1-2.8_