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");
+	});
+});