What is iCalendar and how do you create an ICS file?

With iCalendar, you have the opportunity to combine your appointments from different programs into one format. These can then be managed and modified as files in a standardized format. Additionally, sharing information with friends and business partners becomes much easier — since it follows a global standard, almost anyone can open and use the files.

What is iCalendar? Definition and explanation of the data exchange format

iCalendar is a calendar exchange format for distributing calendar units which is standardized in RFC 5545. This makes it possible to bundle appointments from different applications on your PC and smartphone into a calendar, and to manage and share them from there. The format is based on vCalendar, which was created in 1998. With the iCal format, you can efficiently manage and share all events from various calendar applications in real time and in a clear, structured way.

What does iCal mean?

Depending on the operating system, iCalendar files use different abbreviations — and many people wonder what iCal, ICS, or iFBF actually mean. These are all file extensions for iCalendar: .ics, .ifb, .ical, and .ifbf all refer to files in the iCal format, although there are some differences between them.

• ical: Name extension for macOS
• ics: Filename extension for all other operating systems
• ifbf: Contains information about whether you’re free or busy (for Mac OS X)
• ifb: Contains information whether you are free or busy (all other operating systems)

If you want to send a calendar entry, you need to select the file type of the iCalendar format when saving and sending it. Creating an .ics file is often the best option, as the .ics file structure and format are compatible with most operating systems. When the recipient opens the file, they can then decide whether to accept or decline the appointment.

The benefits of iCalendar

iCalendar is not limited to file exchange. The format behind iCalendar events is closely related to the email standard; it follows the Multipurpose Internet Mail Extension (MIME) protocol, specifically the “text/calendar” type. Thanks to this open design, calendar entries in this format can be placed almost anywhere — even embedded on websites. This makes it easy to share events or publish important dates. Combined with WebDAV servers, which are used to provide files over the internet, appointments can be synchronized at any time.

The format also offers the option to exchange calendar entries directly via Bluetooth or a personal network between mobile devices. So, if you want to schedule the next meeting during a business session, you can simply send the entry to everyone present. In addition, iCalendar allows all stored information to be used across multiple applications, which can be synchronized in real time through the format.

The iCalendar format is accepted by popular calendar programs that integrate the web-based data from the calendar:

• Google Calendar
• Android Calendar
• Microsoft Outlook
• Blackberry Calendar
• macOS/iOS Calendar

What does standardization through RFC 5545 mean?

RFC 5545 defines how calendar information must be structured and encoded so that it can be correctly read and processed by different applications.

At its core is a uniform data structure based on so-called calendar components such as VEVENT, VTODO, VJOURNAL, or VTIMEZONE. Each of these components follows a clearly defined structure with mandatory and optional fields that contain specific properties like the start time, end time, location, or description of an event.

In addition, RFC 5545 also specifies the following characteristics:

• Syntax (UTF-8 text format, line length, line breaks)
• Date and time formats (including time zone information)
• Encoding of recurring events, e.g., through the RRULE property

This ensures that a single .ics file can be interpreted the same way regardless of the software or operating system used.

How to create an iCalendar file

From theory to practice. Making an appointment works as follows:

1. First, open a new document in a text editor of your choice. With the right app, this can also be done on a smartphone.
2. Next, fill the file using the standard iCalendar schema, which we’ll explain in more detail in the next step.
3. After completing the document correctly, export it in the iCal format. To do this, save the file with the extension “.ics.”
4. Open a calendar program of your choice and import the .ics file as an event. A single click on the corresponding menu option is enough, and the event will automatically be uploaded from the directory.
5. The event will now appear as an entry in your calendar.

To ensure the data transfer works properly and the recipient can import the event into their own calendar, the information must be provided in a predefined format. Each line in this format carries a specific meaning:

The time indication also follows a standardized format:

• The first four digits correspond to the year (YYYY): 2019
• The next two, the month (MM): 201910
• And the last two specify the day (DD): 20191027
• You can separate the date from the time with the letter T: 20191027T

The time, in turn, consists of 6 numbers:

+ Two for the hour: 20191027T15 + Two for the minute: 20191027T1559 + And two for the seconds: 20191027T155954 + The date input is closed with a Z: 20191027T155954Z

In the following example all rows are filled out accordingly.

BEGIN:VCALENDAR
VERSION:2.0
PRODID:Cal_App//Daily@Planet
METHOD:PUBLISH
BEGIN:VEVENT
UID:123456789@example.com
LOCATION:Metropolis
SUMMARY:Meeting
DESCRIPTION:Release Meeting
CLASS:PUBLIC
DTSTART:20191101T100000Z
DTEND: 20191101T120000Z
DTSTAMP: 20191027T155954Z
END:VEVENT
END:VCALENDAR

Each recipient of the event can now open the file and have the appointment automatically added to their calendar, receiving all relevant details — from the type of event to the start and end times — completely automatically. This makes the iCalendar format a real time-saver, especially in professional settings.

Embedding iCalendar into the frontend

Instead of creating an ICS file, you can redirect users through special URLs directly to an online calendar provider such as Google Calendar, Outlook.com, or Yahoo Calendar. These links open a new “Add event” window in the browser of the respective service, already pre-filled with details like title, location, description, and start/end time — all based on the iCalendar or iCal format.

<a href="https://calendar.google.com/calendar/render?action=TEMPLATE
&amp;text=Release%20Meeting
&amp;details=Status-Update%20%26%20Milestones
&amp;location=Remote%20(Google%20Meet)
&amp;dates=20251010T080000Z/20251010T090000Z
&amp;ctz=America/New York"
target="_blank" rel="noopener">
Add to Google Calendar
</a>

Generating .ics files with code

Below are practical code examples showing how to create ICS files with just a few lines of code. Each example generates an event called “Release Meeting” on October 15, 2025, from 10:00 to 11:00 America/New York, with proper line endings (CRLF) and simple escaping.

PHP example

In the following PHP example, the helper function ics_escape() is first defined. This function ensures that special characters such as commas, semicolons, backslashes, or line breaks are properly escaped. Then, the structure of the ICS file is defined in an array, with each line representing a specific attribute of the calendar entry, such as the title (SUMMARY), description (DESCRIPTION), or time frame (DTSTART, DTEND). The implode("\r\n", [...]) command joins all lines into one text block.

Next come two header() commands that tell the browser how to handle the file:

• Content-Type: text/calendar; charset=utf-8 indicates that it is a calendar file in the iCalendar format.
• Content-Disposition: attachment; filename="event.ics" instructs the browser not to display the file in the window, but instead to automatically download it with the specified name.

Finally, echo $ics outputs the assembled text. As soon as the PHP script (e.g., event.php) is opened in the browser, the download of the file event.ics starts automatically — the ics calendar file can then be imported directly into Outlook, Google Calendar, or Apple Calendar.

function ics_escape($s) {
    return str_replace(
        ["\\",    ";",    ",",    "\n"],
        ["\\\\","\\;", "\\,", "\\n"],
        $s
    );
}
$ics = implode("\r\n", [
    "BEGIN:VCALENDAR",
    "PRODID:-//example.com//ics-php//US",
    "VERSION:2.0",
    "CALSCALE:GREGORIAN",
    "METHOD:PUBLISH",
    "BEGIN:VEVENT",
    "UID:event-123@example.com",
    "DTSTAMP:20251010T080000Z",
    "DTSTART;TZID=America/New_York:20251015T100000",
    "DTEND;TZID=America/New_York:20251015T110000",
    "SUMMARY:" . ics_escape("Release Meeting"),
    "DESCRIPTION:" . ics_escape("Status Update & Milestones"),
    "LOCATION:" . ics_escape("Remote (Google Meet)"),
    "END:VEVENT",
    "END:VCALENDAR",
    ""
]);
header('Content-Type: text/calendar; charset=utf-8');                // Informs the browser: "This is an .ics file"
header('Content-Disposition: attachment; filename="event.ics"'); // Triggers download
echo $ics; // Sends the text to the browser

Python example

Here, the ics_escape() function also ensures that special characters such as commas, semicolons, or line breaks are correctly escaped. With datetime.now(timezone.utc), a current timestamp (DTSTAMP) is automatically generated in UTC format. The individual lines of the event, such as the title (SUMMARY), are then combined into a text block using "\r\n".join([...]). The CRLF line breaks (\r\n) remain mandatory in the ICS file structure of the iCalendar or iCal format.

The command Path("event-python.ics").write_text(ics, encoding="utf-8", newline="") writes the content as a UTF-8 file into the current directory. After running the script, the finished file event-python.ics is ready and can be imported directly into Outlook, Google Calendar, or Apple Calendar — a practical way to create ICS files programmatically.

from datetime import datetime, timezone
from pathlib import Path
def ics_escape(s: str) -> str:
    return s.replace("\\", "\\\\").replace(";", "\\;").replace(",", "\\,").replace("\n", "\\n")
ics = "\r\n".join([
    "BEGIN:VCALENDAR",
    "PRODID:-//example.com//ics-python//US",
    "VERSION:2.0",
    "CALSCALE:GREGORIAN",
    "METHOD:PUBLISH",
    "BEGIN:VEVENT",
    "UID:event-123@example.com",
    f"DTSTAMP:{datetime.now(timezone.utc):%Y%m%dT%H%M%SZ}",
    "DTSTART;TZID=America/New_York:20251015T100000",
    "DTEND;TZID=America/New_York:20251015T110000",
    f"SUMMARY:{ics_escape('Release Meeting')}",
    f"DESCRIPTION:{ics_escape('Status Update & Milestones')}",
    f"LOCATION:{ics_escape('Remote (Google Meet)')}",
    "END:VEVENT",
    "END:VCALENDAR",
    ""
])
Path("event-python.ics").write_text(ics, encoding="utf-8", newline="")
print("Written: event-python.ics")

JavaScript example

The following JavaScript code generates the calendar content directly in the browser, following the standard ICS file structure of the iCalendar or iCal format. First, the iCalendar text is assembled in the variable ics. The icsEscape() function ensures that special characters like commas or line breaks are correctly escaped. Using Blob(), the script creates a virtual file in memory, and URL.createObjectURL() turns it into a temporary download URL that the browser treats like a real file.

An invisible <a> element with the download attribute is dynamically created and triggered automatically by a.click() — this starts the download of the ICS file event-browser.ics without requiring any server connection. This is a simple way to create iCal or create ICS files directly in the frontend.

<button id="dl">Download .ics file</button>
<script>
function icsEscape(s){
    return s.replace(/\\/g,"\\\\")
                    .replace(/;/g,"\\;")
                    .replace(/,/g,"\\,")
                    .replace(/\n/g,"\\n");
}
const ics = [
    "BEGIN:VCALENDAR",
    "PRODID:-//example.com//ics-browser//US",
    "VERSION:2.0",
    "CALSCALE:GREGORIAN",
    "METHOD:PUBLISH",
    "BEGIN:VEVENT",
    "UID:event-123@example.com",
    "DTSTAMP:20251010T080000Z",
    "DTSTART;TZID=America/New_York:20251015T100000",
    "DTEND;TZID=America/New_York:20251015T110000",
    "SUMMARY:" + icsEscape("Release Meeting"),
    "DESCRIPTION:" + icsEscape("Status Update & Milestones"),
    "LOCATION:" + icsEscape("Remote (Google Meet)"),
    "END:VEVENT",
    "END:VCALENDAR",
    ""
].join("\r\n");
document.getElementById("dl").addEventListener("click", () => {
    const blob = new Blob([ics], { type: "text/calendar;charset=utf-8" }); // Create file object
    const url = URL.createObjectURL(blob); // Temporary download URL
    const a = document.createElement("a");
    a.href = url;
    a.download = "event-browser.ics"; // Download name
    document.body.appendChild(a);
    a.click(); // Start download
    a.remove();
    URL.revokeObjectURL(url); // Cleanup
});
</script>

Node.js example

This code starts a small Express web server that serves an ICS file directly through the browser, following the standard iCalendar or iCal format. When someone accesses http://localhost:3000/event.ics, the app.get(...) handler generates the iCalendar text and sends it to the client.

Just like in previous examples, the icsEscape() function ensures that special characters are properly escaped. With res.setHeader("Content-Type", "text/calendar; charset=utf-8"), the server informs the browser that it’s sending an ics calendar file. The header Cache-Control: public, max-age=300 allows clients to cache the file for 5 minutes.

Finally, res.send(ics) sends the completed calendar entry directly to the browser, which either downloads it or opens it in the default calendar app. The server then runs locally under http://localhost:3000/event.ics. This is a simple and efficient way to create iCal or create ICS files dynamically via Node.js.

import express from "express";
const app = express();
function icsEscape(s){
    return s.replace(/\\/g,"\\\\").replace(/;/g,"\\;").replace(/,/g,"\\,").replace(/\n/g,"\\n");
}
app.get("/event.ics", (req, res) => {
    const ics = [
        "BEGIN:VCALENDAR",
        "PRODID:-//example.com//ics-node//US",
        "VERSION:2.0",
        "CALSCALE:GREGORIAN",
        "METHOD:PUBLISH",
        "BEGIN:VEVENT",
        "UID:event-123@example.com",
        "DTSTAMP:20251010T080000Z",
        "DTSTART;TZID=America/New_York:20251015T100000",
        "DTEND;TZID=America/New_York:20251015T110000",
        "SUMMARY:" + icsEscape("Release Meeting"),
        "DESCRIPTION:" + icsEscape("Status Update & Milestones"),
        "LOCATION:" + icsEscape("Remote (Google Meet)"),
        "END:VEVENT",
        "END:VCALENDAR",
        ""
    ].join("\r\n");
    res.setHeader("Content-Type", "text/calendar; charset=utf-8"); // Browser recognizes MIME type
    res.setHeader("Cache-Control", "public, max-age=300");                    // 5-minute caching
    res.send(ics); // Send response
});
app.listen(3000, () => console.log("http://localhost/event.ics"));

.ics files in different calendar clients

Even though iCalendar is an open standard, each calendar application interprets .ics files in its own way. Depending on the system, there are differences in how events are imported, synchronized, and updated.

Google Calendar

In Google Calendar, users can either manually import ICS files or subscribe to a calendar feed via URL.

When importing, the events contained in the ICS file are added once to the selected calendar. Any changes you later make to the file on your server will not be updated automatically. Google treats this option as a static snapshot of your iCalendar data.

The subscription via URL works a bit differently and is only available in the web version of Google Calendar. Here, you select “Other calendars” → “Add by URL” and enter the web address of an ICS calendar file, for example https://example.com/calendar.ics. Google then fetches this file regularly—usually every 12 to 24 hours—and automatically updates events if any changes are detected. It’s important to note that Google does not accept webcal:// links. If you want to offer a subscription link on your website, you should instead link to the HTTPS version of the URL when you create ICS files for public access.

Apple Calendar

Apple Calendar provides particularly comprehensive and convenient support for the iCalendar format (.ics). When users download or open an ICS file via email, the system automatically recognizes it and asks which calendar the events should be imported into. As with other clients, this is initially a one-time import that is not automatically updated later.

Much more powerful, however, is the subscription feature, which Apple supports directly through the webcal:// scheme. If you offer a link like webcal://example.com/calendar.ics on your website, clicking it automatically opens the calendar dialog asking whether the ICS calendar should be subscribed to. Such a subscribed calendar is then automatically synchronized — typically every few hours. Changes to existing events (same UID, new DTSTAMP) are properly updated, and deleted entries are removed during the next refresh. This makes Apple’s implementation of the iCal format ideal for users who want to create ICS files and share them for ongoing updates.

Outlook

Microsoft Outlook can also import and subscribe to .ics files. When an ICS file is opened by double-clicking or imported manually, Outlook simply adds the included events to the selected calendar. These events are static — later changes on the server do not affect them.

When subscribing, Outlook behaves similarly to Apple Calendar: users can go to “Open Calendar” → “From Internet” and enter the feed’s URL (for example, https://example.com/calendar.ics or webcal://...). Outlook then subscribes to the ICS calendar and automatically synchronizes it at regular intervals. The sync period usually ranges from 30 minutes to 3 hours but can sometimes take longer.

Outlook.com (the web version) works in much the same way technically, although updates there tend to be slower than in the desktop app.