" 3.32 Once a flow is identified, the time at which it occurred must be determined so that the value of all flows within a given accounting period can be compiled.
The international accounts do not show individual transactions or other flows, but there are several reasons why precise rules on their individual timing
must be given. First, rules have to be formulated to determine in which accounting period the discrete flows are to be recorded. Second, an exact timing of individual flows within the accounting period is crucial to make the distinction between changes in net worth due to transactions and those due to other changes (e.g., other changes in volume and revaluations). Third, the integrated nature of the system means that the positions recorded on the balance sheet are influenced by the timing of flows. Finally, the quadruple accounting system requires that entries for a transaction are made by the counterparties at the same time. This ensures the consistency of accounts for each party (e.g., consistency between the merchandise item and the corresponding
financial account entries) as well as the symmetry of recording by partner economies.
3.33 One of the problems in determining the timing of transactions is that activities of institutional units often stretch over periods in which several important moments can be distinguished. For instance, exports and imports of goods commence with the signing of a contract between a seller and a buyer; encompass dates of crossing borders, a date of delivery, and a date or dates
on which payments become due; and are completed only when the last payment is received by the seller. Each of these distinct moments in time is, to some extent, economically relevant and may result in multiple transactions
in the international accounts. As explained in the following paragraphs, each transaction should be recorded according to the accrual basis, which determines the time period to which it should be attributed. "
Para 3.32 - 3.33, Chapter 3, Page 35, BPM6