Spacetime Converter - what's the theory?

[Doctor Q]

From the description of Spacetime Converter ([app=Spacetime Converter]iTunes link[/app]):

Have you thought about how old you are - in meters?

Or how tall you are - in seconds?

Or how many miles it is until your next birthday?

Let's think out of the box, relativistically. While spacetime intervals are constant, length and time can be converted to each other. This calculator makes use of geometrized units (where the speed of light is unity) and lets you convert conveniently between length and time.

Is this a useful conversion tool in your day-to-day life? Probably not. But maybe it will let you think about the world a little differently. And amuse your friends and family at the same time!

For a technical reference, refer to "Gravitation", by Misner, Thorne and Wheeler, 1973, W.H. Freeman and Company, pp. 35-36.​

Here is that book. Do any physics majors have a copy to look this up?

From this applications example (1.74 meters is 5.80402 nanoseconds), they are converting time intervals to the distance light travels in that amount of time. Does this match the concepts of spacetime intervals (the shortest paths between events) and proper time (time as measured by a clock undergoing acceleration)?