refactor: Include ValidityPeriod logic

src/price.ts

@@ -1,10 +1,10 @@
 import { Currency, PriceCurrencyFormat } from "./currency";
 import { approxScaleBigInt } from "./number";
 
-export type Price = {
+export interface Price {
   value: bigint;
   currency: Currency;
-};
+}
 
 // An ExchangeRates object maps different currencies to their rate in USD,
 // which is a number value. One example of an ExchangeRates object would be:

src/time.ts

@@ -134,3 +134,19 @@ export function prettyTimestampDiffFromNow(timestamp: bigint): string {
     return "less than an hour";
   }
 }
+
+export interface ValidityPeriod {
+  validFrom: bigint;
+  validUntil: bigint;
+}
+
+export const createValidityPeriod = (
+  validFrom: bigint,
+  validUntil: bigint
+): ValidityPeriod => {
+  if (validFrom > validUntil)
+    throw new Error(
+      "Error creating ValidityPeriod. validFrom must be less than validUntil"
+    );
+  return { validFrom, validUntil };
+};