A "datum" is the origin of a system of measurement. Conventionally, the datum of measure is zero:
-n -- 0 -- +n
The Reciprocal System is based on motion, a ratio of space to time as a multiplicative inverse, creating a natural datum of unity (1.0 or 1/1), the center of the system of measurement.
1/n -- 1/1 -- n/1
All measurements are made as displacements from the unit speed datum, in either space or time. Zero does not exist in the multiplicative inverse ratio.