Carl is standing in a park 1000 m across. Rohan flies over the park at a very high speed, first passing over the east end of the park, and then passing over the west end. Carl and Rohan are discussing the time interval between when Rohan passed over the east end of the park and when he passed over the west end of the park. Who measured the proper time?
Carl measured the proper time.
There exists a proper time for these events, but neither Carl nor Rohan measured it.
There is no proper time interval for these two events.