perSecond

How many of these units fit into one second.

Every Arrow timestamp vector stores a Long counting this many parts of a second since the epoch, so this is the divisor that splits such a value into whole seconds plus a sub-second remainder — and the multiplier that puts it back together. Shared by the reader (DataFrame.readArrowFeather) and the writer (AnyFrame.arrowWriter) so the two can never disagree about a unit.