diff --git a/app/features/calendar/calendar-utils.test.ts b/app/features/calendar/calendar-utils.test.ts
new file mode 100644
index 000000000..690c62ca5
--- /dev/null
+++ b/app/features/calendar/calendar-utils.test.ts
@@ -0,0 +1,102 @@
+import { describe, expect, it } from "vitest";
+import {
+	calendarEventMaxDate,
+	calendarEventMinDate,
+	closeByWeeks,
+	datesToRegClosesAt,
+	regClosesAtDate,
+	regClosesAtToDisplayName,
+} from "./calendar-utils";
+
+describe("calendar-utils", () => {
+	it("calendarEventMinDate should return a fixed date", () => {
+		expect(calendarEventMinDate()).toEqual(new Date(Date.UTC(2015, 4, 28)));
+	});
+
+	it("calendarEventMaxDate should return a date one year from now", () => {
+		const result = calendarEventMaxDate();
+		const expected = new Date();
+		expected.setFullYear(expected.getFullYear() + 1);
+		expect(result.getFullYear()).toBe(expected.getFullYear());
+	});
+
+	it("regClosesAtDate should return correct date based on closesAt option", () => {
+		const startTime = new Date();
+		expect(regClosesAtDate({ startTime, closesAt: "5min" })).toEqual(
+			new Date(startTime.getTime() - 5 * 60 * 1000),
+		);
+		expect(regClosesAtDate({ startTime, closesAt: "1h" })).toEqual(
+			new Date(startTime.getTime() - 60 * 60 * 1000),
+		);
+	});
+
+	it("regClosesAtToDisplayName should return correct display name", () => {
+		expect(regClosesAtToDisplayName("5min")).toBe("5 minutes");
+		expect(regClosesAtToDisplayName("1h")).toBe("1 hour");
+	});
+
+	it("datesToRegClosesAt should return correct closesAt option based on date difference", () => {
+		const startTime = new Date();
+		expect(
+			datesToRegClosesAt({
+				startTime,
+				regClosesAt: new Date(startTime.getTime() - 5 * 60 * 1000),
+			}),
+		).toBe("5min");
+		expect(
+			datesToRegClosesAt({
+				startTime,
+				regClosesAt: new Date(startTime.getTime() - 60 * 60 * 1000),
+			}),
+		).toBe("1h");
+	});
+
+	it("closeByWeeks should return correct week numbers and years (middle of year)", () => {
+		const result = closeByWeeks({ week: 24, year: 2024 });
+		expect(result).toEqual([
+			{ number: 20, year: 2024 },
+			{ number: 21, year: 2024 },
+			{ number: 22, year: 2024 },
+			{ number: 23, year: 2024 },
+			{ number: 24, year: 2024 }, // <--
+			{ number: 25, year: 2024 },
+			{ number: 26, year: 2024 },
+			{ number: 27, year: 2024 },
+			{ number: 28, year: 2024 },
+		]);
+	});
+
+	it("closeByWeeks should return correct week numbers and years (start of year)", () => {
+		const result = closeByWeeks({ week: 1, year: 2023 });
+		expect(result).toEqual([
+			{ number: 49, year: 2022 },
+			{ number: 50, year: 2022 },
+			{ number: 51, year: 2022 },
+			{ number: 52, year: 2022 },
+			{ number: 1, year: 2023 }, // <--
+			{ number: 2, year: 2023 },
+			{ number: 3, year: 2023 },
+			{ number: 4, year: 2023 },
+			{ number: 5, year: 2023 },
+		]);
+	});
+
+	it("closeByWeeks should return correct week numbers and years (end of year)", () => {
+		const result = closeByWeeks({ week: 52, year: 2024 });
+		expect(result).toEqual([
+			{ number: 48, year: 2024 },
+			{ number: 49, year: 2024 },
+			{ number: 50, year: 2024 },
+			{ number: 51, year: 2024 },
+			{ number: 52, year: 2024 }, // <--
+			{ number: 1, year: 2025 },
+			{ number: 2, year: 2025 },
+			{ number: 3, year: 2025 },
+			{ number: 4, year: 2025 },
+		]);
+	});
+
+	it("closeByWeeks should throw if week is out of range", () => {
+		expect(() => closeByWeeks({ week: 53, year: 2024 })).toThrow();
+	});
+});
diff --git a/app/features/calendar/calendar-utils.ts b/app/features/calendar/calendar-utils.ts
index aac145b88..b76cd0232 100644
--- a/app/features/calendar/calendar-utils.ts
+++ b/app/features/calendar/calendar-utils.ts
@@ -115,3 +115,27 @@ export function datesToRegClosesAt({
 	logger.warn("datesToRegClosesAt: fallback value");
 	return "0";
 }
+
+export function closeByWeeks(args: { week: number; year: number }) {
+	if (args.week < 1 || args.week > 52) {
+		throw new Error("Invalid week number");
+	}
+
+	return [-4, -3, -2, -1, 0, 1, 2, 3, 4].map((week) => {
+		let number = args.week + week;
+		let year = args.year;
+
+		if (number < 1) {
+			number = 52 + number;
+			year--;
+		} else if (number > 52) {
+			number = number - 52;
+			year++;
+		}
+
+		return {
+			number,
+			year,
+		};
+	});
+}
diff --git a/app/features/calendar/routes/calendar.tsx b/app/features/calendar/routes/calendar.tsx
index 114d5fe5c..96b30a1d0 100644
--- a/app/features/calendar/routes/calendar.tsx
+++ b/app/features/calendar/routes/calendar.tsx
@@ -1,7 +1,7 @@
 import type { LoaderFunctionArgs, MetaFunction } from "@remix-run/node";
 import { Link, useLoaderData, useSearchParams } from "@remix-run/react";
 import clsx from "clsx";
-import { addDays, addMonths, subDays, subMonths } from "date-fns";
+import { addMonths, subMonths } from "date-fns";
 import React from "react";
 import { Flipped, Flipper } from "react-flip-toolkit";
 import { useTranslation } from "react-i18next";
@@ -49,6 +49,7 @@ import type { SerializeFrom } from "../../../utils/remix";
 import * as CalendarRepository from "../CalendarRepository.server";
 import { calendarEventTagSchema } from "../actions/calendar.new.server";
 import { CALENDAR_EVENT } from "../calendar-constants";
+import { closeByWeeks } from "../calendar-utils";
 import { Tags } from "../components/Tags";
 
 import "~/styles/calendar.css";
@@ -157,22 +158,6 @@ export const loader = async ({ request }: LoaderFunctionArgs) => {
 	};
 };
 
-function closeByWeeks(args: { week: number; year: number }) {
-	const dateFromWeekNumber = weekNumberToDate(args);
-
-	return [-4, -3, -2, -1, 0, 1, 2, 3, 4].map((week) => {
-		const date =
-			week < 0
-				? subDays(dateFromWeekNumber, Math.abs(week) * 7)
-				: addDays(dateFromWeekNumber, week * 7);
-
-		return {
-			number: dateToWeekNumber(date),
-			year: date.getFullYear(),
-		};
-	});
-}
-
 function fetchEventsOfWeek(args: {
 	week: number;
 	year: number;