@sdxc/icalendar

Read and write iCalendar documents, with recurrence rules and time zones.

RFC 5545 iCalendar is the format every calendar client subscribes to and imports. Events, alarms and time zones are typed; every other component and property (VTODO, VJOURNAL, X- properties) is kept verbatim, so a calendar read and written back loses nothing. The reader is lenient: structural errors fail with their line, while a missing UID or an unreadable value becomes a warning and the property is kept as written. The writer folds lines at 75 octets, escapes TEXT and parameters, and always writes VERSION:2.0.

Installation

npm add @sdxc/icalendar

Fallible functions return a Result from @sdxc/result; it installs alongside this package.

Usage

Serve a feed

A calendar of UTC date-times needs no VTIMEZONE. utc() turns an instant into one.

import { calendarResponse, utc } from "@sdxc/icalendar";

export function feed(): Response {
	return calendarResponse({
		productId: "-//example//status//EN",
		name: "Maintenance windows",
		refreshInterval: { hours: 1 },
		timeZones: [],
		events: [
			{
				uid: "42@status.example.com",
				dtstamp: new Date(),
				start: utc(Date.UTC(2026, 8, 23, 10)),
				end: utc(Date.UTC(2026, 8, 23, 12)),
				summary: "Database upgrade",
				sequence: 1,
				properties: [],
			},
		],
		components: [],
		properties: [],
	});
}

The response is text/calendar; charset=utf-8, and name and refreshInterval are also written as X-WR-CALNAME and X-PUBLISHED-TTL for clients that read only those. Use stringify(calendar) for the text alone.

Read a calendar

import { parse, toInstant } from "@sdxc/icalendar";
import { isFailure } from "@sdxc/result";

let parsed = parse(await response.text());
if (isFailure(parsed)) throw parsed.error; // ICalendarParseError, with .line

let { calendar, warnings } = parsed.data;
for (let event of calendar.events) {
	let start = toInstant(event.start, calendar); // epoch ms, or null for a DATE or floating time
}

Expand a recurrence

import { occurrences } from "@sdxc/icalendar/rrule";
import { unwrap } from "@sdxc/result";

let now = Date.now();
let upcoming = unwrap(occurrences(event, { from: now, to: now + 30 * 86_400_000, calendar }));
// [{ start, end }, ...] in epoch ms, earliest first

Passing calendar resolves the event's TZIDs through its VTIMEZONEs before Intl.

Write a zoned recurring event

A rule that follows a zone's wall clock needs a TZID and its VTIMEZONE, or it drifts by an hour across DST. vtimezone() builds one covering the dates the calendar writes.

import { stringify } from "@sdxc/icalendar";
import { vtimezone } from "@sdxc/icalendar/timezone";
import { unwrap } from "@sdxc/result";

let zone = unwrap(
	vtimezone("Europe/Madrid", { from: Date.UTC(2026, 0, 1), to: Date.UTC(2028, 0, 1) }),
);

let text = stringify({
	productId: "-//example//team//EN",
	timeZones: [zone],
	events: [
		{
			uid: "standup@example.com",
			dtstamp: new Date(),
			start: {
				type: "date-time",
				wall: { year: 2026, month: 1, day: 5, hour: 9, minute: 30, second: 0 },
				zone: { tzid: "Europe/Madrid" },
			},
			duration: { minutes: 15 },
			recurrence: { frequency: "WEEKLY", byDay: [{ weekday: "MO" }, { weekday: "WE" }] },
			alarms: [{ action: "DISPLAY", trigger: { before: { minutes: 5 } }, description: "Standup" }],
			properties: [],
		},
	],
	components: [],
	properties: [],
});

API

@sdxc/icalendar

parse(source: string): Result<ICalendar.Parsed, ICalendarParseError>

Reads the first VCALENDAR into a typed calendar and its warnings, each prefixed with its line. VEVENTs with a readable DTSTART land in events, readable VTIMEZONEs in timeZones, every other component in components, and properties the model does not name (or whose value does not parse) in the owner's properties.

parseAll(source: string): Result<ICalendar.Parsed[], ICalendarParseError>

Every VCALENDAR in the text, for files that concatenate several.

stringify(calendar: ICalendar.Calendar): string

The calendar as CRLF lines. Every TZID a date-time uses needs its VTIMEZONE in timeZones; an event with both end and duration writes DTEND.

toInstant(value: ICalendar.DateValue, calendar?: ICalendar.Calendar): number | null

The epoch milliseconds a DATE-TIME names. A TZID resolves through the calendar's VTIMEZONE first and Intl second; a DATE, a floating time or an unresolvable TZID gives null.

utc(instant: number | Date): ICalendar.DateValue

A UTC DATE-TIME, truncated to whole seconds.

calendarResponse(calendar: ICalendar.Calendar, init?: ResponseInit & { filename?: string }): Response

A text/calendar; charset=utf-8 response, with ; method=… when calendar.method is set. filename makes it an attachment download. Headers in init win.

escapeText(text: string): string, unescapeText(value: string): string

TEXT escaping (\\, \;, \,, \n), for untyped Property values, which are stored as written.

MEDIA_TYPE

"text/calendar".

ICalendarParseError

A structural failure (an unmatched BEGIN/END, a line without :, no VCALENDAR), with the 1-based line it starts on.

@sdxc/icalendar/rrule

parseRecurrence(value: string): Result<ICalendar.RecurrenceRule, RecurrenceRuleError>

Reads an RRULE value, case-insensitively. An unknown or repeated part, an out-of-range number, or COUNT beside UNTIL fails.

stringifyRecurrence(rule: ICalendar.RecurrenceRule): string

The RRULE value for a rule.

occurrences(event: ICalendar.Event, options: OccurrenceOptions): Result<Occurrence[], RecurrenceRuleError>

The occurrences overlapping [from, to), earliest first: DTSTART, the rule's instances and the RDATEs, minus every EXDATE. Each lasts as long as the event: DTEND gives an exact length, DURATION a nominal one (a day across DST keeps its local time), and an all-day event one day. It fails for a TZID nothing resolves or a rule with out-of-range parts.

• options.from: epoch ms, inclusive; occurrences still running then are included

• options.to: epoch ms, exclusive

• options.limit: the most occurrences returned, 1_000 by default

• options.calendar: resolves TZIDs through its VTIMEZONEs before Intl

• options.timeZone: the IANA zone floating times and DATEs are read in, "UTC" by default

RecurrenceRuleError

An RRULE that does not parse, carries out-of-range parts, or cannot be expanded.

@sdxc/icalendar/timezone

vtimezone(tzid: string, span: { from: number; to: number }): Result<ICalendar.TimeZone, TimeZoneError>

A VTIMEZONE for an IANA zone: the observance in force when the span starts, then one per offset change until it ends. Past the span the last offset holds, so the span should cover every date-time the calendar writes in that zone.

TimeZoneError

A zone Intl does not know, or a span that ends before it starts.

@sdxc/icalendar/itip

Each builder returns a calendar with its METHOD set, ready for stringify or calendarPart, or an ITipError naming the constraint the event breaks.

request(event: ICalendar.Event, options: ITip.Options): Result<ICalendar.Calendar, ITipError>

METHOD:REQUEST, inviting the attendees or sending a revision. The event needs a UID, an organizer and at least one attendee. Attendees without participation or rsvp are asked to reply; SEQUENCE is sent as given, so derive it with nextSequence.

cancel(event: ICalendar.Event, options: ITip.CancelOptions): Result<ICalendar.Calendar, ITipError>

METHOD:CANCEL, with SEQUENCE one past the event's. Without options.attendees it cancels the event for everyone; with them it uninvites just those addresses.

reply(event: ICalendar.Event, options: ITip.ReplyOptions): Result<ICalendar.Calendar, ITipError>

METHOD:REPLY from options.attendee with options.participation (and an optional comment), echoing the request's UID, SEQUENCE and RECURRENCE-ID.

readReply(source: string | ICalendar.Calendar): Result<ITip.Reply[], ITipError>

One entry per VEVENT of a METHOD:REPLY, so a reply to several instances lists each with its recurrenceId. Another method, or a VEVENT without a UID or exactly one ATTENDEE, fails.

nextSequence(previous: ICalendar.Event, next: ICalendar.Event): number

One more than previous.sequence when the start, end, duration, recurrence, RDATE, EXDATE or status changed, the same otherwise.

calendarPart(calendar: ICalendar.Calendar, options?: { filename?: string }): ITip.CalendarPart

{ method, content, filename? }: the calendar's method (PUBLISH when unset) and its text, the shape @sdxc/mail's calendar option takes.

ITipError

An event that cannot carry the method asked of it, or a message that is not a REPLY.

Types

ICalendar

A namespace of the model's types, exported from @sdxc/icalendar:

• Calendar: productId, method?, name?, description?, url?, refreshInterval?, timeZones, events, components, properties

• Event: uid, dtstamp, start, end? or duration?, summary?, description?, location?, url?, status?, transparency?, sequence?, created?, lastModified?, categories?, recurrence?, recurrenceDates?, exceptionDates?, recurrenceId?, organizer?, attendees?, alarms?, properties

• DateValue: { type: "date", year, month, day } or { type: "date-time", wall, zone }, where zone is "utc", "floating" or { tzid }; WallClock holds the wall fields

• Period: { type: "period", start, end?, duration? }, an RDATE with its own length

• Duration: negative?, weeks?, days? (nominal), hours?, minutes?, seconds? (exact)

• RecurrenceRule: frequency, interval?, count? or until?, bySecond? through bySetPosition?, weekStart?; Weekday is "MO" through "SU"

• Alarm: action, trigger ({ before, related? } or { at }), description?, summary?, attendees?, repeat?, properties?; before: { minutes: 15 } fires 15 minutes early

• CalendarUser, Attendee: address (a mailto: URI), name?, parameters?; an attendee adds role?, participation?, rsvp?

• TimeZone, Observance: tzid and its observances, each with kind, local start, offsetFrom/offsetTo in minutes, name?, recurrence?, recurrenceDates?

• Component, Property: the generic layer, { name, properties, components } and { name, parameters, value } with the value as written

• Parsed: { calendar, warnings }

OccurrenceOptions, Occurrence

The options of occurrences() and each { start, end } it returns, from @sdxc/icalendar/rrule.

ITip

From @sdxc/icalendar/itip: Options (productId, dtstamp?, timeZones?), CancelOptions (adds attendees?), ReplyOptions (adds attendee, participation, comment?), Reply (uid, sequence, dtstamp, recurrenceId?, organizer?, attendee) and CalendarPart.

Pattern: Invite, Reschedule and Cancel by Mail

Each message is a request or cancel sent as a calendar part; the recipient's client matches them by UID and keeps the highest SEQUENCE.

import type { ICalendar } from "@sdxc/icalendar";

import { utc } from "@sdxc/icalendar";
import { calendarPart, cancel, nextSequence, request } from "@sdxc/icalendar/itip";
import { unwrap } from "@sdxc/result";

let productId = "-//example//meetings//EN";
let event: ICalendar.Event = {
	uid: "review-7@example.com",
	dtstamp: new Date(),
	start: utc(Date.UTC(2026, 9, 20, 15)),
	end: utc(Date.UTC(2026, 9, 20, 16)),
	summary: "Design review",
	sequence: 0,
	organizer: { address: "mailto:host@example.com", name: "Host" },
	attendees: [{ address: "mailto:guest@example.com", role: "REQ-PARTICIPANT" }],
	properties: [],
};

await mailer.send({
	to,
	subject,
	html,
	calendar: calendarPart(unwrap(request(event, { productId }))),
});

let moved = {
	...event,
	start: utc(Date.UTC(2026, 9, 21, 15)),
	end: utc(Date.UTC(2026, 9, 21, 16)),
};
moved.sequence = nextSequence(event, moved);
await mailer.send({
	to,
	subject,
	html,
	calendar: calendarPart(unwrap(request(moved, { productId }))),
});

await mailer.send({
	to,
	subject,
	html,
	calendar: calendarPart(unwrap(cancel(moved, { productId }))),
});

Pattern: Answer an Invitation and Record the Answer

The attendee's side builds a reply from the event it received; the organizer's side reads it back.

import { parse, stringify } from "@sdxc/icalendar";
import { readReply, reply } from "@sdxc/icalendar/itip";
import { isFailure, unwrap } from "@sdxc/result";

let [invitation] = unwrap(parse(await attachment.text())).calendar.events;
let answer = unwrap(
	reply(invitation, {
		productId: "-//example//client//EN",
		attendee: "mailto:guest@example.com",
		participation: "ACCEPTED",
		comment: "See you there",
	}),
);
let text = stringify(answer);

let replies = readReply(text);
if (isFailure(replies)) throw replies.error;
for (let { uid, sequence, attendee } of replies.data) {
	await saveAnswer(uid, attendee.address, attendee.participation, sequence);
}

A reply whose sequence is lower than the event's current one answers a revision the attendee has since been sent again; compare the two before trusting it.

Pattern: A Feed and a Single-Event Download

Both serve the same events with the same UIDs, so a client holding the feed and the download shows one event. A SEQUENCE that only grows makes clients replace their copy on every edit.

import type { ICalendar } from "@sdxc/icalendar";

import { calendarResponse, utc } from "@sdxc/icalendar";

interface Window {
	id: string;
	name: string;
	startsAt: number;
	endsAt: number;
	createdAt: number;
	updatedAt: number;
}

function toEvent(window: Window): ICalendar.Event {
	return {
		uid: `${window.id}@status.example.com`,
		dtstamp: new Date(window.updatedAt),
		lastModified: new Date(window.updatedAt),
		sequence: Math.floor((window.updatedAt - window.createdAt) / 1000),
		start: utc(window.startsAt),
		end: utc(window.endsAt),
		summary: window.name,
		properties: [],
	};
}

function calendarOf(events: ICalendar.Event[]): ICalendar.Calendar {
	return {
		productId: "-//example//status//EN",
		timeZones: [],
		events,
		components: [],
		properties: [],
	};
}

export function feed(windows: Window[]): Response {
	return calendarResponse(calendarOf(windows.map(toEvent)));
}

export function download(window: Window): Response {
	return calendarResponse(calendarOf([toEvent(window)]), { filename: "maintenance.ics" });
}

Pattern: The Last Day of Every Month

BYMONTHDAY=31 skips short months; a set position over the candidate days picks each month's last.

import type { ICalendar } from "@sdxc/icalendar";

import { occurrences, parseRecurrence, stringifyRecurrence } from "@sdxc/icalendar/rrule";
import { unwrap } from "@sdxc/result";

let rule: ICalendar.RecurrenceRule = {
	frequency: "MONTHLY",
	byMonthDay: [28, 29, 30, 31],
	bySetPosition: [-1],
};
stringifyRecurrence(rule); // "FREQ=MONTHLY;BYMONTHDAY=28,29,30,31;BYSETPOS=-1"
unwrap(parseRecurrence("freq=monthly;bymonthday=-1")); // the same days, written as one negative day

let closing: ICalendar.Event = {
	uid: "close-books@example.com",
	dtstamp: new Date(),
	start: { type: "date", year: 2026, month: 1, day: 31 },
	recurrence: rule,
	properties: [],
};
let dates = unwrap(
	occurrences(closing, {
		from: Date.UTC(2026, 0, 1),
		to: Date.UTC(2027, 0, 1),
		timeZone: "America/New_York",
	}),
);

All-day dates are read in timeZone, so each occurrence runs from local midnight to the next.

Pattern: Is Anything Active Now

A one-millisecond window with limit: 1 answers whether any occurrence, recurring or not, is running.

import type { ICalendar } from "@sdxc/icalendar";

import { occurrences } from "@sdxc/icalendar/rrule";
import { isSuccess } from "@sdxc/result";

function isActive(event: ICalendar.Event, calendar: ICalendar.Calendar, now = Date.now()): boolean {
	let result = occurrences(event, { from: now, to: now + 1, limit: 1, calendar });
	return isSuccess(result) && result.data.length > 0;
}

Pattern: Edit a Calendar Without Losing What It Holds

Components and properties outside the typed model ride along in components and properties, so a read, edit and write keeps them. Untyped values are stored as written; unescapeText reads them.

import { escapeText, parse, stringify, unescapeText } from "@sdxc/icalendar";
import { unwrap } from "@sdxc/result";

let { calendar } = unwrap(parse(source));

let todos = calendar.components.filter((component) => component.name === "VTODO");
let timeZone = calendar.properties.find((property) => property.name === "X-WR-TIMEZONE");

calendar.properties.push({ name: "X-EXAMPLE-NOTE", parameters: {}, value: escapeText("a, b; c") });
calendar.events = calendar.events.map((event) => ({
	...event,
	sequence: (event.sequence ?? 0) + 1,
}));

let text = stringify(calendar); // VTODOs, X-WR-TIMEZONE and the note written back
let note = unescapeText(calendar.properties.at(-1)?.value ?? ""); // "a, b; c"