Duty is set by the law in force on the date goods entered, not the date of the shipment, order, or lookup.
US duty liability attaches on the date goods are entered for consumption — not the date they were ordered, shipped, or invoiced, and not the date someone looks up the rate. Because tariff rates, trade-remedy actions, and exclusions change over time (a Section 232 action can start or a Section 301 exclusion can expire mid-year), the correct duty on a past entry depends on the law that was actually in force on that entry's specific date, which can differ from today's rate. A duty determination that ignores this and simply applies the current rate to a past entry can be materially wrong in either direction.
General principle of US customs law; the operative date is set by the specific proclamation, Federal Register notice, or statute governing each duty program.
For a specific entry's rate, the free Tariff Calculator gives you the stacked, cited answer.