refactor: enforce frontend view model boundaries

src/shared/lib/calendar.ts

@@ -0,0 +1,12 @@
+import { calendarDate } from "./calendar.schema";
+
+function isValidPeriod(from: null | string, to: null | string) {
+	return (
+		[from, to].every(
+			date => date === null || calendarDate.safeParse(date).success,
+		)
+		&& (!from || !to || from <= to)
+	);
+}
+
+export { isValidPeriod };