Some of these quantities refer directly to a point of time. That is true of "capital value" as also of such quantities as demand and supply prices. Other terms – as e. g. "income", "revenue", "return", "expenses", "savings", "investments" – imply, however, a time period for which they are reckoned. But in order to be unambiguous they must also refer to a point of time at which they are calculated.

p. 46-7 - Monetary Equilibrium, (1939)