Model Timestamps and Rate Updates in a Delivery-Fee System

In an object-oriented delivery-fee system, explain how you would represent timestamps and update rates. Describe the responsibilities and data structures needed to apply the correct rate consistently when rates change. The detailed fee formula and starter code are not supplied. Do not assume a per-minute, distance-based, or concurrent-order formula; make the timing and update policy explicit before using it. ### Constraints & Assumptions - The design must explain timestamp choice and rate updates. - Currency precision, effective-time semantics, and retroactive changes require clarification. - Preserve old calculation evidence when a rate configuration changes. ### Clarifying Questions to Ask - Which event determines the applicable rate: order creation, service start, completion, or each segment of service? - Does an update apply only prospectively, and can effective times be backdated? - Are displayed local times converted to unambiguous instants before storage? ```hint Overwriting one current-rate field makes it difficult to explain which rate an earlier calculation used. ``` ### What a Strong Answer Covers Separate rate policy from fee calculation, use an explicit time model, and preserve versioned effective-rate information. Explain concurrent updates and how a calculation binds to the chosen policy without inventing the missing fee formula. ### Follow-up Questions - What happens when two rate changes have the same effective instant? - How would you audit a fee after the current rate changes?