diff --git a/web/src/components/events/EventContainer.tsx b/web/src/components/events/EventContainer.tsx
index 6965883..2534f95 100644
--- a/web/src/components/events/EventContainer.tsx
+++ b/web/src/components/events/EventContainer.tsx
@@ -26,13 +26,12 @@ import {
 import { isTodayOrFuture } from "@/lib/date";
 import { useToday } from "@/components/general/Today";
 import styles from "./eventContainer.module.scss";
-import { formatDate, formatYearMonth } from "@/lib/date";
 import {
-  startOfWeek,
-  endOfWeek,
-  differenceInCalendarDays,
-  parse,
-} from "date-fns";
+  calendarWeekPadding,
+  formatDay,
+  formatYearMonth,
+  parseDay,
+} from "@/lib/date";
 import { Icon } from "../general/Icon";
 import { useState } from "react";
 import { VenueFragment } from "@/gql/graphql";
@@ -191,7 +190,7 @@ const CalendarDay = ({
   <div
     className={`${styles.calendarDay} ${events.length === 0 && styles.empty}`}
   >
-    <h3>{formatDate(parse(day, "yyyy-MM-dd", new Date(0)), "eeee dd.MM.")}</h3>
+    <h3>{formatDay(day, "eeee dd.MM.")}</h3>
     <ul>
       {events.map((event) => (
         <EventItem
@@ -210,15 +209,8 @@ const CalendarWeek = ({
 }: {
   days: Record<string, SingularEvent<EventOverviewItemFragment>[]>;
 }) => {
-  const daysInWeek = Object.keys(days);
-  const firstDay = daysInWeek[0];
-  const lastDay = daysInWeek[daysInWeek.length - 1];
-
-  const weekStart = startOfWeek(firstDay, { weekStartsOn: 1 });
-  const weekEnd = endOfWeek(lastDay, { weekStartsOn: 1 });
-
-  const emptyCellsBefore = differenceInCalendarDays(firstDay, weekStart);
-  const emptyCellsAfter = differenceInCalendarDays(weekEnd, lastDay);
+  const { before: emptyCellsBefore, after: emptyCellsAfter } =
+    calendarWeekPadding(Object.keys(days));
 
   return (
     <div className={styles.calendarWeek}>
@@ -237,7 +229,7 @@ const CalendarWeek = ({
 
 function srOnlyYear(yearMonthString: string, today: Date) {
   // formatYearMonth only shows the year for non-current years; always expose it to screen readers
-  const yearMonth = new Date(yearMonthString);
+  const yearMonth = parseDay(yearMonthString);
   const isCurrentYear = yearMonth.getFullYear() === today.getFullYear();
   if (!isCurrentYear) {
     return null;
diff --git a/web/src/lib/date.ts b/web/src/lib/date.ts
index 6ea566c..20c6a34 100644
--- a/web/src/lib/date.ts
+++ b/web/src/lib/date.ts
@@ -1,10 +1,13 @@
 import {
+  differenceInCalendarDays,
+  endOfWeek,
+  format as formatWithoutTimeZone,
   isBefore,
   parse,
+  startOfWeek,
   compareAsc,
   addDays,
   isSameDay,
-  startOfDay,
 } from "date-fns";
 import { nb } from "date-fns/locale";
 import { toZonedTime, format, formatInTimeZone } from "date-fns-tz";
@@ -26,9 +29,43 @@ export function osloDay(now: Date = new Date()): string {
   return formatInTimeZone(now, timeZone, "yyyy-MM-dd");
 }
 
+// Two kinds of Date are used here. An instant is a real point in time (what
+// the CMS sends). A wall-clock Date has its own fields set to what a clock in
+// Oslo shows, whatever timezone the code runs in: toLocalTime() makes one
+// from an instant, parseDay() from a day string. Convert once, then stay on
+// wall-clock Dates: formatDate() takes instants, formatWallClock() the rest
+
+// "yyyy-MM-dd" or "yyyy-MM" as a wall-clock Date (midnight, first of month)
+export function parseDay(day: string): Date {
+  return parse(day, day.length === 7 ? "yyyy-MM" : "yyyy-MM-dd", new Date(0));
+}
+
+export function formatWallClock(date: Date, formatStr: string) {
+  return formatWithoutTimeZone(date, formatStr, { locale: nb });
+}
+
+// Formats a "yyyy-MM-dd" or "yyyy-MM" string
+export function formatDay(day: string, formatStr: string) {
+  return formatWallClock(parseDay(day), formatStr);
+}
+
 // Start of an Oslo day, comparable with toLocalTime() values
 export function startOfOsloDay(day: string): Date {
-  return parse(day, "yyyy-MM-dd", new Date(0));
+  return parseDay(day);
+}
+
+// How many empty cells a calendar week needs before its first and after its
+// last day ("yyyy-MM-dd", in order) to span mandag–søndag
+export function calendarWeekPadding(days: string[]) {
+  const first = parseDay(days[0]);
+  const last = parseDay(days[days.length - 1]);
+  return {
+    before: differenceInCalendarDays(
+      first,
+      startOfWeek(first, { weekStartsOn: 1 })
+    ),
+    after: differenceInCalendarDays(endOfWeek(last, { weekStartsOn: 1 }), last),
+  };
 }
 
 export function formatDate(date: Date | string | number, formatStr: string) {
@@ -37,11 +74,11 @@ export function formatDate(date: Date | string | number, formatStr: string) {
 
 export function formatYearMonth(yearMonth: string, today: Date) {
   // full name of month if year is current year, otherwise name of month + year
-  const parsed = parse(yearMonth, "yyyy-MM", new Date(0));
+  const parsed = parseDay(yearMonth);
   if (parsed.getFullYear() === today.getFullYear()) {
-    return formatDate(parsed, "MMMM");
+    return formatWallClock(parsed, "MMMM");
   }
-  return formatDate(parsed, "MMMM yyyy");
+  return formatWallClock(parsed, "MMMM yyyy");
 }
 
 export function formatExtendedDateTime(
@@ -76,16 +113,15 @@ export function isConsecutiveDays(a: Date, b: Date): boolean {
 
 export function groupConsecutiveDates(dates: string[]): string[][] {
   const groupedDates: string[][] = [];
-  const uniqueDays = unique(
-    dates.map((date) => format(startOfDay(toLocalTime(date)), "yyyy-MM-dd"))
+  const uniqueDays: string[] = unique(
+    dates.map((date) => formatWallClock(toLocalTime(date), "yyyy-MM-dd"))
   ).sort();
   let group: string[] = [];
 
   for (let i = 0; i < uniqueDays.length; i++) {
     if (
       i === 0 ||
-      isSameDay(uniqueDays[i - 1], uniqueDays[i]) ||
-      isConsecutiveDays(uniqueDays[i - 1], uniqueDays[i])
+      isConsecutiveDays(parseDay(uniqueDays[i - 1]), parseDay(uniqueDays[i]))
     ) {
       group.push(uniqueDays[i]);
     } else {
@@ -125,38 +161,39 @@ export function formatOccurrenceMonths(starts: string[]): string {
 
   return groups
     .map((g) => {
-      const first = parse(g[0], "yyyy-MM", new Date(0));
+      const first = parseDay(g[0]);
       if (g.length === 1) {
-        return formatDate(first, "MMMM yyyy");
+        return formatWallClock(first, "MMMM yyyy");
       }
-      const last = parse(g[g.length - 1], "yyyy-MM", new Date(0));
+      const last = parseDay(g[g.length - 1]);
       const firstFmt =
         first.getFullYear() === last.getFullYear()
-          ? formatDate(first, "MMMM")
-          : formatDate(first, "MMMM yyyy");
-      return `${firstFmt} – ${formatDate(last, "MMMM yyyy")}`;
+          ? formatWallClock(first, "MMMM")
+          : formatWallClock(first, "MMMM yyyy");
+      return `${firstFmt} – ${formatWallClock(last, "MMMM yyyy")}`;
     })
     .join(", ");
 }
 
+// Takes "yyyy-MM-dd" days in order, as grouped by groupConsecutiveDates
 export function formatDateRange(dates: string[]): string {
   if (dates.length === 1) {
-    return formatDate(dates[0], "d. MMMM");
+    return formatDay(dates[0], "d. MMMM");
   }
 
-  const firstDate = toLocalTime(dates[0]);
-  const lastDate = toLocalTime(dates[dates.length - 1]);
+  const firstDate = parseDay(dates[0]);
+  const lastDate = parseDay(dates[dates.length - 1]);
 
   if (firstDate.getMonth() === lastDate.getMonth()) {
     // same month
-    return `${firstDate.getDate()}.—${lastDate.getDate()}. ${formatDate(
+    return `${firstDate.getDate()}.—${lastDate.getDate()}. ${formatWallClock(
       firstDate,
       "MMMM"
     )}`;
   }
 
   // continues into next month
-  const formattedFirstDate = formatDate(firstDate, "d. MMMM");
-  const formattedLastDate = formatDate(lastDate, "d. MMMM");
+  const formattedFirstDate = formatWallClock(firstDate, "d. MMMM");
+  const formattedLastDate = formatWallClock(lastDate, "d. MMMM");
   return `${formattedFirstDate}—${formattedLastDate}`;
 }
diff --git a/web/src/lib/event.ts b/web/src/lib/event.ts
index ed39fbf..82bb13f 100644
--- a/web/src/lib/event.ts
+++ b/web/src/lib/event.ts
@@ -8,7 +8,7 @@ import {
 } from "date-fns";
 import {
   toLocalTime,
-  formatDate,
+  formatWallClock,
   compareDates,
 } from "./date";
 import { graphql, unmaskFragment } from "@/gql";
@@ -266,28 +266,30 @@ export function organizeEventsInCalendar<E extends EventListable>(
 
   const calendar: EventCalendar<E> = {};
 
-  const minDate = new Date(sortedEvents[0]?.occurrence.start);
-  const maxDate = new Date(
+  // wall-clock dates from here on: weeks and days are Oslo's, not the
+  // runtime's
+  const minDate = toLocalTime(sortedEvents[0]?.occurrence.start);
+  const maxDate = toLocalTime(
     sortedEvents[sortedEvents.length - 1]?.occurrence.start
   );
 
   let currentDate = startOfWeek(minDate, { weekStartsOn: 1 });
   while (currentDate <= maxDate) {
-    const week = formatDate(currentDate, "w");
+    const week = formatWallClock(currentDate, "w");
     const daysOfWeek = eachDayOfInterval({
       start: currentDate,
       end: endOfWeek(currentDate, { weekStartsOn: 1 }),
     });
 
     daysOfWeek.forEach((day) => {
-      const yearMonth = formatDate(day, "yyyy-MM");
+      const yearMonth = formatWallClock(day, "yyyy-MM");
       if (!calendar[yearMonth]) {
         calendar[yearMonth] = {};
       }
       if (!calendar[yearMonth][week]) {
         calendar[yearMonth][week] = {};
       }
-      const formattedDay = formatDate(day, "yyyy-MM-dd");
+      const formattedDay = formatWallClock(day, "yyyy-MM-dd");
       if (!calendar[yearMonth][week][formattedDay]) {
         calendar[yearMonth][week][formattedDay] = [];
       }
@@ -298,9 +300,9 @@ export function organizeEventsInCalendar<E extends EventListable>(
 
   sortedEvents.forEach((event) => {
     const start = toLocalTime(event.occurrence.start);
-    const yearMonth = formatDate(start, "yyyy-MM");
-    const week = formatDate(start, "w");
-    const day = formatDate(start, "yyyy-MM-dd");
+    const yearMonth = formatWallClock(start, "yyyy-MM");
+    const week = formatWallClock(start, "w");
+    const day = formatWallClock(start, "yyyy-MM-dd");
     if (!calendar[yearMonth]) {
       calendar[yearMonth] = {};
     }
@@ -341,7 +343,7 @@ export function organizeEventsByDate<E extends EventListable>(
 
   sortedEvents.forEach((event) => {
     const start = toLocalTime(event.occurrence.start);
-    const day = formatDate(start, "yyyy-MM-dd");
+    const day = formatWallClock(start, "yyyy-MM-dd");
     if (!eventsByDate[day]) {
       eventsByDate[day] = [];
     }
diff --git a/web/src/lib/timezone.test.ts b/web/src/lib/timezone.test.ts
new file mode 100644
index 0000000..0a33e7b
--- /dev/null
+++ b/web/src/lib/timezone.test.ts
@@ -0,0 +1,140 @@
+import { afterEach, describe, expect, it } from "vitest";
+
+import {
+  calendarWeekPadding,
+  formatDateRange,
+  formatDay,
+  formatOccurrenceMonths,
+  formatYearMonth,
+  groupConsecutiveDates,
+  isTodayOrFuture,
+  parseDay,
+  startOfOsloDay,
+} from "./date.ts";
+import {
+  getFutureOccurrences,
+  getSingularEvents,
+  organizeEventsByDate,
+  organizeEventsInCalendar,
+} from "./event.ts";
+
+// Much of this code also runs in the visitor's browser, in whatever timezone
+// that is set to. Everything shown is Oslo time, so the result must not
+// depend on the runtime's timezone
+const TIMEZONES = [
+  "Europe/Oslo",
+  "UTC",
+  "Asia/Tokyo",
+  "Pacific/Auckland",
+  "America/New_York",
+  "America/Los_Angeles",
+];
+
+const originalTz = process.env.TZ;
+afterEach(() => {
+  process.env.TZ = originalTz;
+});
+
+const makeEvent = (id: string, starts: string[]) => ({
+  id,
+  occurrences: starts.map((start, i) => ({ id: `${id}-${i}`, start, end: null })),
+});
+
+// 00:30 on onsdag 1. juli 2026 in Oslo; still 30. juni in UTC and the Americas
+const JUST_PAST_MIDNIGHT = "2026-06-30T22:30:00Z";
+
+describe.each(TIMEZONES)("with the runtime in %s", (tz) => {
+  const inTz = (name: string, fn: () => void) =>
+    it(name, () => {
+      process.env.TZ = tz;
+      fn();
+    });
+
+  inTz("formats day and month strings as written", () => {
+    expect(formatDay("2026-07-01", "eeee dd.MM.")).toBe("onsdag 01.07.");
+    expect(formatDay("2026-01", "MMMM yyyy")).toBe("januar 2026");
+    expect(parseDay("2026-01").getFullYear()).toBe(2026);
+    expect(parseDay("2026-01-01").getDate()).toBe(1);
+  });
+
+  inTz("formats month headings", () => {
+    const today = startOfOsloDay("2026-07-07");
+    expect(formatYearMonth("2026-07", today)).toBe("juli");
+    expect(formatYearMonth("2026-01", today)).toBe("januar");
+    expect(formatYearMonth("2027-01", today)).toBe("januar 2027");
+  });
+
+  inTz("formats occurrence months and date ranges", () => {
+    expect(formatOccurrenceMonths([JUST_PAST_MIDNIGHT])).toBe("juli 2026");
+    expect(
+      formatOccurrenceMonths(["2026-12-05T12:00:00Z", "2027-01-05T12:00:00Z"])
+    ).toBe("desember 2026 – januar 2027");
+    expect(formatDateRange(["2026-07-01"])).toBe("1. juli");
+    expect(formatDateRange(["2026-07-01", "2026-07-03"])).toBe("1.—3. juli");
+    expect(formatDateRange(["2026-06-30", "2026-07-01"])).toBe(
+      "30. juni—1. juli"
+    );
+  });
+
+  inTz("groups consecutive Oslo days", () => {
+    expect(
+      groupConsecutiveDates([
+        JUST_PAST_MIDNIGHT,
+        "2026-07-02T18:00:00Z",
+        "2026-07-04T18:00:00Z",
+      ])
+    ).toEqual([["2026-07-01", "2026-07-02"], ["2026-07-04"]]);
+    // across the 25-hour DST fall-back day
+    expect(
+      groupConsecutiveDates([
+        "2026-10-24T18:00:00Z",
+        "2026-10-25T18:00:00Z",
+        "2026-10-26T18:00:00Z",
+      ])
+    ).toEqual([["2026-10-24", "2026-10-25", "2026-10-26"]]);
+  });
+
+  inTz("places events on their Oslo day in the calendar", () => {
+    const events = getSingularEvents([makeEvent("a", [JUST_PAST_MIDNIGHT])]);
+
+    const calendar = organizeEventsInCalendar(events);
+
+    // the seeded week starts mandag 29. juni; June holds only padding
+    expect(Object.keys(calendar)).toEqual(["2026-07"]);
+    const [week] = Object.values(calendar["2026-07"]);
+    expect(Object.keys(week)).toEqual([
+      "2026-07-01",
+      "2026-07-02",
+      "2026-07-03",
+      "2026-07-04",
+      "2026-07-05",
+    ]);
+    expect(week["2026-07-01"].map((e) => e.id)).toEqual(["a"]);
+
+    expect(Object.keys(organizeEventsByDate(events))).toEqual(["2026-07-01"]);
+  });
+
+  inTz("pads a partial calendar week to mandag and søndag", () => {
+    // onsdag 1. – søndag 5. juli
+    expect(
+      calendarWeekPadding(["2026-07-01", "2026-07-02", "2026-07-05"])
+    ).toEqual({ before: 2, after: 0 });
+    // mandag 27. – tirsdag 30. juni
+    expect(calendarWeekPadding(["2026-06-29", "2026-06-30"])).toEqual({
+      before: 0,
+      after: 5,
+    });
+  });
+
+  inTz("judges past and future by the Oslo day", () => {
+    const today = startOfOsloDay("2026-07-01");
+    expect(isTodayOrFuture(JUST_PAST_MIDNIGHT, today)).toBe(true);
+    expect(isTodayOrFuture("2026-06-30T21:30:00Z", today)).toBe(false);
+    expect(
+      getFutureOccurrences(
+        makeEvent("a", ["2026-06-30T21:30:00Z", JUST_PAST_MIDNIGHT]),
+        today
+      ).map((o) => o.start)
+    ).toEqual([JUST_PAST_MIDNIGHT]);
+  });
+});