refactor: simplify calendar and form state

src/shared/lib/calendar.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, it } from "vitest";
+
+import { isValidPeriod, moscowDate } from "./calendar";
+import { calendarDate } from "./calendar.schema";
+
+describe("calendar dates", () => {
+	it.each([
+		["0000-02-29", true],
+		["0001-01-01", true],
+		["0099-12-31", true],
+		["1900-02-29", false],
+		["2000-02-29", true],
+		["2024-02-29", true],
+		["2026-02-29", false],
+		["2026-99-99", false],
+		["2026-1-01", false],
+		["", false],
+	])("validates %s without Date normalization", (value, valid) => {
+		expect(calendarDate.safeParse(value).success).toBe(valid);
+	});
+
+	it("accepts open or single-day periods and rejects reversed ranges", () => {
+		expect(isValidPeriod(null, null)).toBe(true);
+		expect(isValidPeriod(null, "2026-09-01")).toBe(true);
+		expect(isValidPeriod("2026-09-01", null)).toBe(true);
+		expect(isValidPeriod("2026-09-01", "2026-09-01")).toBe(true);
+		expect(isValidPeriod("2026-09-02", "2026-09-01")).toBe(false);
+		expect(isValidPeriod("", "2026-09-01")).toBe(false);
+	});
+
+	it("changes the calendar day at Moscow midnight across a year boundary", () => {
+		expect(moscowDate(new Date("2026-12-31T20:59:59Z"))).toBe("2026-12-31");
+		expect(moscowDate(new Date("2026-12-31T21:00:00Z"))).toBe("2027-01-01");
+	});
+});

src/shared/lib/calendar.ts

@@ -9,4 +9,18 @@ function isValidPeriod(from: null | string, to: null | string) {
 	);
 }
 
-export { isValidPeriod };
+function moscowDate(value: Date): string {
+	const parts = new Intl.DateTimeFormat("en-CA", {
+		day: "2-digit",
+		month: "2-digit",
+		timeZone: "Europe/Moscow",
+		year: "numeric",
+	}).formatToParts(value);
+
+	const part = (type: string) =>
+		parts.find(item => item.type === type)?.value;
+
+	return `${part("year")}-${part("month")}-${part("day")}`;
+}
+
+export { isValidPeriod, moscowDate };

src/shared/lib/index.ts

@@ -1,4 +1,4 @@
-export { isValidPeriod } from "./calendar";
+export { isValidPeriod, moscowDate } from "./calendar";
 export { calendarDate } from "./calendar.schema";
 export {
 	formatChartValue,