1. An on demand net accrual system for tracking accrued revenue and costs, comprising: a computer including an accrual system that causes each accrual event to be posted at the same time that the accrual event was received wherein an accrual event includes: ordered items that have been partially delivered but remain unbilled; ordered items that have been shipped, but not yet billed; and ordered items that have been billed to a client, but not yet posted, a reversal system that, on demand, checks a set of reversal rules and reverses each accrual event that triggered a reversal rule and, an accrual tracking table for storing accrual events, wherein the reversal system causes accrual events to be removed from the accrual tracking table and added to an accrual history table when the accrual event is reversed.