It is implicitly, but this is often obscured because currency is both a medium of exchange and a store of value. Accounting makes the distinction between your income statement (which has a time period attached, usually either quarterly or annually, and is focused on dollars exchanged with other entities) and your balance sheet (which is a snapshot quantity at a particular point in time, and is focused on dollars stored). GDP is part of a country's income statement - it reflects how many goods & services were produced & sold within the country.