Guide

ICS (iCalendar) Format: Digital Calendar Events & Scheduling

PC By Pablo Cirre • Updated May 7, 2026

What Is ICS (iCalendar)?

The ICS format (file extension .ics) is the standard file format for storing and exchanging calendar data. Based on the iCalendar specification (RFC 5545), it is a plain-text format readable by every major calendar application — Google Calendar, Microsoft Outlook, Apple Calendar, Mozilla Thunderbird, and hundreds of scheduling tools.

History of the iCalendar Standard

The iCalendar standard was developed by the Internet Engineering Task Force (IETF) and first published as RFC 2445 in November 1998. It was designed to standardize calendar data interchange across different applications and platforms, replacing the earlier vCalendar 1.0 standard created by the Versit Consortium (the same group behind vCard).

In 2009, RFC 2445 was superseded by RFC 5545, which clarified ambiguities, corrected inconsistencies, and added improved timezone handling. A companion specification, RFC 5546, defines the iTIP (iCalendar Transport-Independent Interoperability Protocol) — the mechanism used for sending meeting invitations via email.

Today iCalendar is universally supported: Google Calendar, Microsoft Outlook, Apple Calendar, Thunderbird, Fastmail, Proton Calendar, Calendly, Cal.com, and virtually every scheduling application reads and writes .ics files.

Technical Structure of an ICS File

ICS files are plain text, encoded in UTF-8, using a line-by-line PROPERTY:VALUE format. Every file begins with BEGIN:VCALENDAR and ends with END:VCALENDAR.

Top-Level Properties

VERSION:2.0 — iCalendar version (always 2.0 for RFC 5545)
PRODID — identifier of the software that created the file
CALSCALE:GREGORIAN — calendar system (Gregorian is the default)
METHOD — used in email invitations (PUBLISH, REQUEST, REPLY, CANCEL)

Components

Component	Description
`VEVENT`	A calendar event (meeting, appointment, holiday)
`VTODO`	A task or to-do item with optional due date
`VJOURNAL`	A journal entry associated with a date
`VFREEBUSY`	Free/busy time information for scheduling
`VTIMEZONE`	Timezone definition referenced by events

Key VEVENT Properties

Property	Description
`UID`	Unique identifier for the event
`DTSTART`	Start date and time
`DTEND`	End date and time
`SUMMARY`	Event title
`DESCRIPTION`	Detailed description
`LOCATION`	Physical or virtual location
`ORGANIZER`	Event organizer (email address)
`ATTENDEE`	Invited attendee with RSVP status
`RRULE`	Recurrence rule (for repeating events)
`VALARM`	Reminder/alarm component

Complete Example VEVENT

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//KaijuConverter//Calendar//EN
BEGIN:VEVENT
UID:meeting-2025-04-25@kaijuconverter.com
DTSTART:20250425T140000Z
DTEND:20250425T150000Z
SUMMARY:Team Meeting
DESCRIPTION:Weekly sync with the team
LOCATION:Conference Room A
ORGANIZER:mailto:organizer@example.com
ATTENDEE;RSVP=TRUE:mailto:colleague@example.com
BEGIN:VALARM
TRIGGER:-PT15M
ACTION:DISPLAY
DESCRIPTION:Meeting starts in 15 minutes
END:VALARM
END:VEVENT
END:VCALENDAR

Recurrence Rules (RRULE)

One of the most powerful features of iCalendar is the RRULE property, which defines repeating events using a compact syntax.

# Every Monday, Wednesday, Friday until end of 2025
RRULE:FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=20251231T000000Z

# Monthly on the last Friday
RRULE:FREQ=MONTHLY;BYDAY=-1FR

# Every 2 weeks on Tuesday
RRULE:FREQ=WEEKLY;INTERVAL=2;BYDAY=TU

# Annually (yearly anniversary)
RRULE:FREQ=YEARLY

RRULE parameters:

FREQ — DAILY, WEEKLY, MONTHLY, YEARLY
INTERVAL — repeat every N periods (default 1)
BYDAY — specific days of the week (MO, TU, WE, TH, FR, SA, SU)
UNTIL — end date for the recurrence
COUNT — maximum number of occurrences

Timezones in ICS: The Tricky Part

Timezone handling is the most common source of ICS import bugs. ICS supports three approaches:

1. UTC time (recommended for unambiguous events):

DTSTART:20250425T140000Z

The Z suffix means UTC. This is unambiguous — every calendar app will show the correct local time.

2. Named timezone (for events that should follow local time):

DTSTART;TZID=Europe/Madrid:20250425T160000

Requires a matching VTIMEZONE block in the file, or the app must recognize the timezone name.

3. Floating time (no timezone — dangerous):

DTSTART:20250425T160000

No Z, no TZID. Each calendar app interprets this as the user's local time, which causes events to appear at the wrong time for attendees in different timezones.

Best practice: Always use UTC (Z) for events shared across timezones. An event at 14:00 UTC is 10:00 AM New York, 15:00 London, 16:00 Madrid — calendar apps automatically convert for each user.

What ICS Files Can Contain

Single events — a one-time meeting or appointment
Full calendar exports — all events from an entire calendar
Recurring events — meetings that repeat daily, weekly, monthly, or yearly
All-day events — use DATE type instead of DATETIME (DTSTART;VALUE=DATE:20250425)
Events with URLs — link to an online meeting or document
Attendee lists — with RSVP status (ACCEPTED, DECLINED, TENTATIVE)
Alarms/reminders — VALARM component triggers at a specified time before the event
Tasks — VTODO component with optional due dates and completion status

Subscribing vs. Importing ICS

These are two fundamentally different operations:

	Import (file)	Subscribe (URL)
How	Open a `.ics` file	Add a `webcal://` or `https://` URL
Updates	One-time snapshot	Live-synced, auto-refreshes
Use case	Conference agenda, one-time events	Sports schedules, public holidays, colleague calendars
Changes	Never propagate	Automatically reflected

When to use subscribing: public sports schedules, TV show release dates, colleagues' shared calendars, public holiday calendars.

When to use importing: conference schedules, one-time events received from a contact, archived calendar data.

Where ICS Files Are Used

Email meeting invitations — every "Add to Calendar" button in Gmail/Outlook generates and attaches an .ics file
Google Calendar export — Settings → Import & Export → Export creates a .ics for each calendar
Outlook calendar sharing — File → Save Calendar generates an .ics file
Public calendar subscriptions — sports team schedules, bank holidays, conference programs
Scheduling apps — Calendly, Cal.com, and Acuity Scheduling send .ics confirmation files to both organizer and attendee
Airline/hotel bookings — many booking confirmation emails include an .ics attachment for the trip dates
Project management tools — Jira, Asana, Linear can export sprint deadlines as ICS calendars

How to Import ICS Files

Google Calendar

Go to calendar.google.com on desktop (import is not available in the mobile app)
Click the gear icon → Settings
Navigate to Import & Export → Import
Select your .ics file and choose which calendar to import into
Click Import

Tip: If you received an .ics via Gmail as an attachment, Gmail shows an "Add to Calendar" button directly — no manual import needed.

Microsoft Outlook

File → Open & Export → Import/Export
Select Import an iCalendar (.ics) or vCalendar file
Browse to your .ics file and click OK
Choose to open as a new calendar or import into an existing one

Apple Calendar

File → Import
Select your .ics file
Choose which calendar to add the events to

Universal shortcut: Most calendar apps on desktop also support simply double-clicking an .ics file — the operating system opens it in the default calendar app and prompts for import.

Creating ICS Files Programmatically

Minimal ICS Template (text editor)

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//KaijuConverter//Calendar//EN
BEGIN:VEVENT
UID:unique-id@yourdomain.com
DTSTART:20250425T140000Z
DTEND:20250425T150000Z
SUMMARY:Team Meeting
DESCRIPTION:Weekly sync with the team
LOCATION:Conference Room A
END:VEVENT
END:VCALENDAR

Save with .ics extension and recipients can open it in any calendar app.

Python (icalendar library)

pip install icalendar

from icalendar import Calendar, Event
from datetime import datetime, timezone

cal = Calendar()
cal.add('prodid', '-//KaijuConverter//Calendar//EN')
cal.add('version', '2.0')

event = Event()
event.add('summary', 'Team Meeting')
event.add('dtstart', datetime(2025, 4, 25, 14, 0, 0, tzinfo=timezone.utc))
event.add('dtend', datetime(2025, 4, 25, 15, 0, 0, tzinfo=timezone.utc))
event.add('description', 'Weekly sync with the team')
event['uid'] = 'meeting-2025-04-25@kaijuconverter.com'

cal.add_component(event)

with open('meeting.ics', 'wb') as f:
    f.write(cal.to_ical())

Converting ICS Files

ICS to CSV — extract event data (title, start, end, location) into spreadsheet rows for analysis
ICS to PDF — create a printable calendar or event agenda
ICS to JSON — for web application consumption
Multiple ICS files to one — merge several calendar exports into a single file

Use KaijuConverter to convert .ics files to CSV, JSON, or other formats instantly.

Related conversions

Frequent conversions across the catalogue:

Frequently Asked Questions

On desktop: go to calendar.google.com → click the gear icon → Settings → Import & Export → Import → select your .ics file → choose which calendar to import into → click Import. On mobile, you can't import ICS directly from the Google Calendar app — use the desktop method. If you received an ICS via email as an attachment, Gmail will show an 'Add to Calendar' button directly in the email.

On desktop: go to calendar.google.com → click the gear icon → Settings → Import & Export → Import → select your .ics arquivo → choose which calendar to import em → click Import. On mobile, you can't import ICS directly de the Google Calendar app — usar the desktop method. If you received an ICS via email as an attachment, Gmail will show an 'Add to Calendar' button directly no email.

On desktop: go to calendar.google.com → click the gear icon → Settings → Import & Export → Import → select your .ics Datei → choose which calendar to import in → click Import. On mobile, you can't import ICS directly von the Google Calendar app — verwenden the desktop method. If you received an ICS via email as an attachment, Gmail will show an 'Add to Calendar' button directly im email.

On desktop: go to calendar.google.com → click the gear icon → Settings → Import & Export → Import → select your .ics archivo → choose which calendar to import en → click Import. On mobile, you can't import ICS directly de the Google Calendar app — usar the desktop method. If you received an ICS via email as an attachment, Gmail will show an 'Add to Calendar' button directly en el email.

On KaijuConverter every file is processed inside an isolated container, encrypted in transit (TLS 1.3) and at rest, and automatically deleted after 60 minutes with multi-pass overwrite. We never train on, share, or analyze user content. For maximum privacy on extremely sensitive material, prefer offline tools (ImageMagick, FFmpeg, LibreOffice) that you control end-to-end.

Importing (opening a .ics file) creates a one-time snapshot of the events — they're added to your calendar but won't automatically update if the source changes. Subscribing to a calendar via a webcal:// or https:// URL creates a live-synced calendar that automatically refreshes. Use subscribing for dynamic calendars (sports schedules, public holidays, colleagues' availability); use importing for one-time event imports like a conference schedule.

Importing (opening a .ics file) creates a one-time snapshot of the events — they're added to your calendar mas won't automatically update if the source changes. Subscribing para um calendar via a webcal:// ou https:// URL creates a live-synced calendar that automatically refreshes. usar subscribing para dynamic calendars (sports schedules, public holidays, colleagues' availability); usar importing para one-time event imports like a conference schedule.

Importing (opening a .ics file) creates a one-time snapshot des events — they're added to your calendar aber won't automatically update if the source changes. Subscribing zu einem calendar via a webcal:// oder https:// URL creates a live-synced calendar that automatically refreshes. verwenden subscribing für dynamic calendars (sports schedules, public holidays, colleagues' availability); verwenden importing für one-time event imports like a conference schedule.

Importing (opening a .ics file) creates a one-time snapshot del events — they're added to your calendar pero won't automatically update if the source changes. Subscribing a un calendar via a webcal:// o https:// URL creates a live-synced calendar that automatically refreshes. usar subscribing para dynamic calendars (sports schedules, public holidays, colleagues' availability); usar importing para one-time event imports like a conference schedule.

For 95% of use cases, yes — server-side ImageMagick, FFmpeg and LibreOffice produce identical output to the same tools on your laptop. Desktop software wins for: extremely large files (multi-GB), batch jobs of thousands of files, scripted pipelines, or content too sensitive to upload. KaijuConverter caps at 25 MB per file on the free tier (up to 2 GB on paid plans).

Most calendar apps create ICS files automatically when you invite attendees — Google Calendar and Outlook both send .ics attachments in meeting invitation emails. To create one manually: open a text editor, paste the VCALENDAR template, fill in DTSTART (format: YYYYMMDDTHHMMSSZ for UTC), DTEND, SUMMARY, DESCRIPTION, and LOCATION, save with .ics extension. Recipients can open it in any calendar app. For programmatic creation, use Python's icalendar library (pip install icalendar).

Most calendar apps create ICS arquivos automatically when you invite attendees — Google Calendar e Outlook both enviar .ics attachments in meeting invitation emails. To create one manually: abrir a text editor, paste the VCALENDAR template, fill in DTSTART (format: YYYYMMDDTHHMMSSZ para UTC), DTEND, SUMMARY, DESCRIPTION, e LOCATION, salvar com .ics extension. Recipients can abrir it in any calendar app. para programmatic creation, usar Python's icalendar library (pip install icalendar).

Most calendar apps create ICS Dateien automatically when you invite attendees — Google Calendar und Outlook both senden .ics attachments in meeting invitation emails. To create one manually: öffnen a text editor, paste the VCALENDAR template, fill in DTSTART (format: YYYYMMDDTHHMMSSZ für UTC), DTEND, SUMMARY, DESCRIPTION, und LOCATION, speichern mit .ics extension. Recipients can öffnen it in any calendar app. für programmatic creation, verwenden Python's icalendar library (pip install icalendar).

Most calendar apps create ICS archivos automatically when you invite attendees — Google Calendar y Outlook both enviar .ics attachments in meeting invitation emails. To create one manually: abrir a text editor, paste the VCALENDAR template, fill in DTSTART (format: YYYYMMDDTHHMMSSZ para UTC), DTEND, SUMMARY, DESCRIPTION, y LOCATION, guardar con .ics extension. Recipients can abrir it in any calendar app. para programmatic creation, usar Python's icalendar library (pip install icalendar).

Most format conversions are lossy by design — JPG, MP3, MP4, WebP all discard perceptual data to save bytes. Going through a lossy intermediate compounds the loss. To minimize visible/audible drift: convert from the original master, choose a higher quality setting, and avoid converting back and forth between lossy formats.

Timezone issues are the most common ICS problem. An ICS event set at 14:00 UTC is 2pm London time, 10am New York time, 7am Los Angeles time — if your calendar app displays it at 2pm in your local time, the ICS probably used floating time (no timezone specified) and your app interpreted it as local time. Check the DTSTART line: if it ends with Z it's UTC; if it has TZID= it's a named timezone; if neither, it's floating. When creating ICS files, always use explicit UTC times (ending in Z) to avoid ambiguity.

Timezone issues are the most common ICS problem. An ICS event set at 14:00 UTC is 2pm London time, 10am New York time, 7am Los Angeles time — if your calendar app displays it at 2pm in your local time, the ICS probably used floating time (no timezone specified) e your app interpreted it as local time. Check the DTSTART line: if it ends com Z it's UTC; if it has TZID= it's a named timezone; if neither, it's floating. When creating ICS files, always usar explicit UTC times (ending in Z) to avoid ambiguity.

Timezone issues are the most common ICS problem. An ICS event set at 14:00 UTC is 2pm London time, 10am New York time, 7am Los Angeles time — if your calendar app displays it at 2pm in your local time, the ICS probably used floating time (no timezone specified) und your app interpreted it as local time. Check the DTSTART line: if it ends mit Z it's UTC; if it has TZID= it's a named timezone; if neither, it's floating. When creating ICS files, always verwenden explicit UTC times (ending in Z) to avoid ambiguity.

Timezone issues are the most common ICS problem. An ICS event set at 14:00 UTC is 2pm London time, 10am New York time, 7am Los Angeles time — if your calendar app displays it at 2pm in your local time, the ICS probably used floating time (no timezone specified) y your app interpreted it as local time. Check the DTSTART line: if it ends con Z it's UTC; if it has TZID= it's a named timezone; if neither, it's floating. When creating ICS files, always usar explicit UTC times (ending in Z) to avoid ambiguity.

Yes — KaijuConverter accepts multiple files in a single drop and returns a ZIP. For very large batches (thousands of files) consider command-line tools or our API: <code>find . -name "*.heic" -exec magick {} {.}.jpg \;</code> or similar one-liners scale to millions of files when run locally.