Add to Calendar Time Zones iCal Library

Convert the IANA (formerly Olson) time zone database files into VTIMEZONE blocks, compatible with the iCalendar specification (RFC2445).

It is based on vzic, but only provides the VTIMEZONE part to easily put this into any other iCal generator (like the add-to-calendar-button).
It is built to be used standalone, via API, or as JavaScript/TypeScript module, hosted as npm package.

📦 Installation / Setup

0. Update it

This step is only relevant if you want to build the package and data on your own and from source!
(Requires Node.js as well as Linux as operating system. You would also need to load/include it manually into your codebase.)

Run ...

sudo sh scripts/update-tzdata.sh tzVersionNumber rebuildVzic

• with tzVersionNumber being the version of the IANA time zone database you want to use (e.g. sudo sh scripts/update-tzdata.sh 2025c);
• and rebuildVzic as an optional param to rebuild the vzic library from scratch (e.g. sudo sh scripts/update-tzdata.sh 2025c true).

1. Install it

npm install timezones-ical-library

2. Import it

For CommonJS:

require tzLib = require('timezones-ical-library');

With ES:

import { tzlib_get_ical_block, tzlib_get_offset, tzlib_get_timezones } from 'timezones-ical-library';

3. Use it

A. Get time zones

Use tzlib_get_timezones() to retrieve a list of all available time zone strings.
You can pass true to retrieve a JSON formatted string instead of an array.
For CommonJS, you would use tzLib.tzlib_get_timezones().

B. Get the iCal time zone block

Use the tzlib_get_ical_block(tzName) function to return the proper iCal VTIMEZONE block for a given time zone string (tzName).
Again, pass true to retrieve a JSON formatted string instead of an array (not recommended).
For CommonJS, you would use tzLib.tzlib_get_ical_block(tzName).

You will receive an array, holding the VTIMEZONE block first, and the TZID line (additionally) second. The latter one is needed for any further time statement.

Include this into your iCal data to come up with a complete ics file.

A final constellation could look like this:

  BEGIN:VCALENDAR
  VERSION:2.0
  PRODID:-// github.com/add2cal/add-to-calendar-button //EN
  CALSCALE:GREGORIAN
+ BEGIN:VTIMEZONE
+ TZID:America/New_York
+ LAST-MODIFIED:20220824T133813Z
+ X-LIC-LOCATION:America/New_York
+ BEGIN:DAYLIGHT
+ TZNAME:EDT
+ TZOFFSETFROM:-0500
+ TZOFFSETTO:-0400
+ DTSTART:19700308T020000
+ RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
+ END:DAYLIGHT
+ BEGIN:STANDARD
+ ZNAME:EST
+ TZOFFSETFROM:-0400
+ TZOFFSETTO:-0500
+ DTSTART:19701101T020000
+ RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
+ END:STANDARD
+ END:VTIMEZONE
  BEGIN:VEVENT
  UID:2022-08-30T19:29:38.618Z@add-to-calendar-button
  DTSTAMP:20230214T091500Z
  DTSTART;TZID=America/New_York:20230214T091500
  DTEND;TZID=America/New_York:20230218T223000
  SUMMARY:A sample event
  DESCRIPTION:Just some descriptive text...
  LOCATION:World Wide Web
  STATUS:CONFIRMED
  LAST-MODIFIED:20220830T192938Z
  SEQUENCE:0
  END:VEVENT
  END:VCALENDAR

C. Get a specific offset

Use tzlib_get_offset(tzName, isoDate, isoTime) to get specific offset (relative to UTC), based on a provided date and time.

For example, you can provide tzName 'Europe/Berlin', isoDate '2023-05-23', and isoTime '15:45' in order to retrieve the offset, which applies for this time zone at the 23rd of May in 2023 at exactly 15:45.

⚙️ API

Instead of using this code or the npm package, you could also make use of the free and open API.

GET a list of all zone names

https://tz.add-to-calendar-technology.com/api/zones.json

GET the ics part of a respective zone

https://tz.add-to-calendar-technology.com/api/{{zoneName}}.ics

With {{zoneName}} being your time zone.
So, for New York, this would be https://tz.add-to-calendar-technology.com/api/America/New_York.ics.

This is case sensitive!

[!WARNING] Mind that this does not deliver a fully valid ics file, since it only contains the VTIMEZONE part. You will need to combine this with your other event information.

🙌 Contributing

Anyone is welcome to contribute, but mind the guidelines:

• Bug reports
• Feature requests
• Pull requests

📃 Copyright and License

Copyright (c) Jens Kuerschner. Licensed under Apache-2.0.

⚡ Changelog

• v2.0 : Full refactoring reducing bundle size and easing future updates, using Astro for Demo page
• v1.11 : database update (2025c)
• v1.10 : upgrading to latest vzic improvements
• v1.9 : database update (2025b)
• v1.8 : database update (2024a)
• v1.7 : database update (2023c)
• v1.6 : database update (2022g)
• v1.5 : new dist structure
• v1.4 : further minification via data mapping
• v1.3 : bundle size optimization and API
• v1.2 : providing tzblock as array with separate tzid
• v1.1 : get-offset function
• v1.0 : initial release

💜 Kudos go to

• The "Time Zone King" Paul Eggert for optimizing time zones for the web, ...
• as well as all the contributors at the official tz repository.
• The awesome authors of zic, vzic, and libical; mainly Damon Chaplin, Allen Winter, and Ken Murchison.