How Google Calendar works

Someone emailed us complaining that we were sending him notifications of events a year in the future, and asking us to stop. Since we do not send email notifications of that kind, this sounded strange, and worrying in that we cannot stop doing something that we weren’t doing in the first place!

The result of the investigation may be useful to someone else, which is why we are posting it here.

The emails in question

Here is a sample of the emails that the person was receiving:

From: <sdf97w4ebsd8hbdseh443djd@import.calendar.google.com>
Date: Thu, 21 Mar 2019, 04:02
Subject: New event: Friday of the 3rd week of Lent @ Fri 20 Mar 2020
To: <ambrose@mulliner.com>

Friday of the 3rd week of Lent
http://universalis.com/20200320/today.htm

When: Fri 20 Mar 2020
Calendar: sdf97w4ebsd8hbdseh443djd@import.calendar.google.com

Invitation from Google Calendar

You are receiving this email at the account ambrose@mulliner.com because you are subscribed for new event updates on calendar sdf97w4ebsd8hbdseh443djd@import.calendar.google.com.

To stop receiving these emails, please log in to https://www.google.com/calendar/ and change your notification settings for this calendar.

Where the calendar events were coming from

Whoever owned the Google Calendar in question had asked it to subscribe to events from the Universalis calendar.

What this means is that every few days Google Calendar visits the link at the bottom of our calendar page, and retrieves the list of events it provides. Each visit provides all liturgical days for exactly a year ahead.

This is straightforward. Either you want those events in your Google Calendar or you do not. If you do, you tell it to subscribe to Universalis. If you change your mind later on, you tell it to unsubscribe. Google make this simple.

Where the Google notifications were coming from

What was puzzling to our correspondent was that he was getting almost daily emails reporting liturgical dates, and those dates were a year ahead. He was understandably annoyed and wanted us to stop. We could not stop, since we were not sending those emails, but at least we were able to work out what he had done.

It turns out that if you own (or subscribe to) a Google calendar, you can ask that calendar to send you a notification every time an event is added. This is sensible if, for instance, you share a Home calendar with your family, or have a Work calendar in your office, and want to see when people have added things to it.

The trouble is that if you tell a Google calendar to tell you when events have been added, it obeys you. It tells you when events have been added. So what was happening here was this:

• On 21 March 2019, the Google calendar retrieved 366 days’ worth of events from our page, going from 21 March 2019 to 20 March 2020, inclusive.
• Because of previous visits, the calendar already had all the events up to and including 19 March 2020.
• So the event of 20 March 2020 (Friday of the 3rd week of Lent, as it happens) was new.
• The Google calendar knew that it had to send notifications of new events.
• So the Google calendar sent a notification of a new future event on 20 March 2020.

Everything was duly correct and exactly as instructed, but it was puzzling, confusing and annoying!

What could be done about it

First of all, we couldn’t do anything. We weren’t sending any emails: a Google calendar was. The Universalis web site was a purely passive partner in all this. It was asked by Google, “Give me the next year’s worth of events”, and it obeyed.

The person who was being annoyed by the notifications had three options.

1. Stop this calendar from subscribing to Universalis events. This is easily done by logging in to Google Calendar. Of course, the calendar will then not have liturgical days in it.
2. Stop this calendar from sending emails every time an event is added. This is also done by logging in to Google Calendar. Of course, the whole calendar will then not report when events are added, wherever those events may come from.
3. Set up a separate Google Calendar to subscribe to Universalis events. This is what you should do if you want to see the Universalis events, and you want to receive notifications of changes to your main calendar, but you don’t want to be told the liturgical days a year ahead. Your main calendar can carry on notifying, but it doesn’t get the Universalis events; your extra calendar will get the Universalis events, but will not notify you about them.

We don’t know which change our correspondent made in the end. But if you run into the same trouble, you now know what to do.

And even if you don’t, it stands as an example of how to perfectly straightforward Google Calendar features can combine to produce an effect which is not straightforward at all.