In theoretical physics, a chronometric singularity (also called a temporal or horological singularity) is a point at which time cannot be measured or described.
An example involves a time at a coordinate singularity, e.g. a geographical pole.
Since time on Earth is measured through longitudes, and no unique longitude exists at a pole, time is not defined uniquely at this point.
In relativity, similar singularities can be found in the case of Schwarzschild coordinates.
Stephen Hawking once compared by a talk-show guest's question about "before the beginning of time" to asking "what's north of the North Pole".