Calendars and scheduling software

June 25, 2015 — July 14, 2023


There are two problems I would like to solve with event scheduling:

  1. The technical end of getting calendars and reminders on my phone and desktop machine
  2. The social problem of discovering and coordinating events with my contacts

That’s the raw technical end; there is also the question of social event discovery. What is the social platform that allows us to collectively discover events? Facebook has made a bid to be a monopoly provider of these services, about which I feel negative.

Jargon to know : CalDAV and CardDAV are the de facto standards to sync your calendar and contact information (respectively) in a machine readable form. Note, however that they are not actually implemented in a standardised way, preferring the chaos of stupid approach; See Alessandro Rossini, The sad story of the vCard format and its lack of interoperability.

There is also some standard that some mail providers seem to use where contacts and calendars are stored in mailboxes; I do not know how that works or what it is called.

Another sometimes-useful keyword is “groupware” which AFAICT is a buzzword left over from the early 2000s which meant “collective calendars+contacts+email+other cruft”, which is approximately what we want I suppose, but only enterprise procurement staff can say it with a straight face, because saying the skill of saying “groupware” unironically is an interview question when recruiting for such roles.

1 GUI Calendar clients for Linux

See calendar clients for linux.

2 Calendar client code libraries

You want to do something really geeky with your caldav and some server. OK.

3 Value-add scheuling services

3.1 Minetime

Not quite a GUI but an interface nonetheless, minetime uses machine learning to optimise appointments/scheduling etc.

3.2 Scheduling meetings

The next two are calendar automation/integration tools.

There are more on the various online listicles to this theme. One can alternatively just use any old survey tool such as google forms or jotforms.

4 Calendar servers

So who gets to have your previous calendar and contacts data? Which jurisdiction gets to harvest your contacts? Do you trust ’em? Should you run your own server? That’s a legitimate use of your precious free time, right? I do not have really good answers in terms of trading off confidentiality versus convenience.

Practically, the only contact syncing that works well with iOs is iCloud. They claim to support any CardDAV server, but in practice I have had glitches and failures with all other servers that I have tried. iCloud is compatible with non-Apple clients, so if you are an Apple user and you trust them enough the convenience is worth it.

4.1 iCloud

iCloud setup for non-apple clients is easy, or requires a few steps, depending on the client.

Log in to!&page=signin (as opposed to the icloud page) to set up an app password. Then you can use that password plus the usual icloud username to work with other clients. As we learn from the 2017 Blackberry docs (!), CardDAV URL is and calendar URL is If the caldav client does not negotiate the correct server correctly, there is a complicated manual setup to try.

There is an iOS app Contact mover which brute-force synchronises icloud contacts to other CardDAV servers.

4.2 Outlook


4.3 Apple Calendar Server

Apple’s open-source Calendar Server. Does contacts too. I don’t think that Apple any longer dogfoods this particular bit of software, but it might work OK. It is bloated.

4.4 Radicale

macOS/Windows/Linux: Radicale seems to be less bloated than the Apple thingy.

Radicale is a small but powerful CalDAV (calendars, todo-lists) and CardDAV (contacts) server, that:

  • Shares calendars through CalDAV, WebDAV and HTTP.
  • Shares contacts through CardDAV, WebDAV and HTTP.
  • Supports events, todos, journal entries and business cards.
  • Works out-of-the-box, no installation nor configuration required. […]
  • Is GPLv3-licensed free software.

4.5 DAVmail


Ever wanted to get rid of Outlook? DavMail is a POP/IMAP/SMTP/CalDAV/CardDAV/LDAP exchange gateway allowing users to use any mail/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the internet or behind a firewall through Outlook Web Access.

DavMail also supports the CardDAV protocol to sync address books. This new feature is sponsored by French Defense / DGA through project Trustedbird. […]

DavMail gateway is implemented in java and should run on any platform. Releases are tested on Windows, Linux (Ubuntu) and macOS. Tested successfully with the Iphone (gateway running on a server).

4.6 Nextcloud calendar plugin

nextcloud, the file sync server, also optionally supports CardDAV and a Calendar GUI.

4.7 Kolab

kolab is the open-source backend of, e.g., kolabnow and used by such apps as kube.

4.8 Sogo


SOGo is a fully supported and trusted groupware server with a focus on scalability and open standards. SOGo is released under the GNU GPL/LGPL v2 and above.

SOGo provides a rich AJAX-based Web interface and supports multiple native clients through the use of standard protocols such as CalDAV, CardDAV and GroupDAV, as well as Microsoft ActiveSync.

SOGo is the missing component of your infrastructure; it sits in the middle of your servers to offer your users a uniform and complete interface to access their information. It has been deployed in production environments where thousands of users are involved.

4.9 iRedMail

iRedMail is a packaged system for some of these projects

With iRedMail, you can deploy an OPEN SOURCE, FULLY FLEDGED, FULL-FEATURED mail server in several minutes, for free.

We did the heavy lifting of putting all the open source components together and applying best practices. Our product does all the major tasks for you. Furthermore we offer professional support to back you up in case you have some problems. […] Manage your calendars (CalDAV), address books (CardDAV), tasks on a easy to use web UI or your mobile devices (iOS, Android, BlackBerry 10, Windows Phone).

In particular the calendar/contacts server comes from sogo.

5 Social event discovery

This one is the real weak spot. Facebook has had an effective monopoly on this in the west.

Calgator is an open source community calendar platform written in Ruby on Rails that runs, a Portland tech calendar.

Gigtripper looks interesting. creates calendar links.

I built after I had multiple clients request the exact same thing — a simple and easy way to generate calendar links, for adding to their website or in email newsletters.

There’s a few existing providers out there, but they’re extremely pricey for what they do — take some basic data (title/date/etc) and reformat it into a url for a calendar provider (be it google or apple).

6 Bonus: Calendars that are not Gregorian