1. A portfolio accounting system adapted to handle uncertainty in accounting information due to time delay in information issuance, wherein the accounting information is represented by streams of sequenced simple transaction states, each simple transaction state comprising at least four attributes, among which a first attribute corresponds to a value date, a second attribute corresponds to a reporting date, a third attribute corresponds to a value parameter and a fourth attribute is an account reference, said value date being a date said value parameter refers to and said reporting date being a date or time said value date and value parameter together with their account reference are entered into or are deemed to have become known to the portfolio accounting system, while each stream links all consecutive simple transaction states representing views of the same accounting information as known to the system over time.