Why an ICS subscription (.ics link) isn't enough to sync your calendars

Updated October 2026 · the Calen1 team

In short: An ICS subscription isn't a sync: it's another calendar reading a copy, one-way, read-only, and it can take hours to refresh. It's fine for looking at someone else's calendar; if you need what you change in one to change in the other within minutes, you need a real sync.

It's the first tip in every forum: “copy your Google .ics link and paste it into Outlook”. And it works, in the sense that your events show up. The trouble comes a few days later, when you move an appointment and the other calendar still shows the old time, or when you try to edit from there and it won't let you.

You're not doing it wrong. An ICS subscription isn't a sync: it's another calendar reading a copy of yours every so often. For some things it's perfect; for keeping your work and your life in order, it falls short in four specific ways.

This guide explains exactly what an ICS link is, how it differs from importing a file and from syncing, and how to tell which one you need.

What an ICS link is (and what it isn't)

ICS, or iCalendar, is a file format for calendar events that almost every app understands. An ICS link, also called an iCal address or calendar URL, is a web address where a file with your events lives. There are two ways to use it, and people mix them up all the time:

Importing an .ics file. You download the file and load it into the other calendar. The events are copied once. Whatever changes afterwards doesn't travel, and if you import again to catch up, everything gets duplicated.
Subscribing to the link. The other calendar stores the address and re-reads the file every so often. It updates on its own, but it's still just a read: what you see is a separate, read-only calendar.

In the first case the link to the original is lost; in the second, it only goes one way. That's exactly what a sync solves.

How it's used in Google Calendar and Outlook

To be clear about what we mean by “the ICS link”:

To get the link from Google Calendar: open the calendar's settings and, in the section for integrating it, copy the secret address in iCal format.
To subscribe from Outlook: in Outlook on the web, add a calendar using the option to subscribe from the web and paste the link.
To subscribe from Google Calendar: under “Other calendars”, use the plus sign and choose “From URL”.
To get a link from Outlook: Outlook on the web lets you publish a calendar and choose whether the link shows only when you're busy or more detail, if your company hasn't blocked it.

Menu names change now and then, but the idea is the same in every version: one app publishes an address and the other reads it.

The four limits of an ICS link

It goes one way. The subscribed calendar only receives. Whatever you create on the other side doesn't go back to the original, so keeping both up to date would take two crossed subscriptions, and each side would still be read-only.
You can't edit it. You can't move or delete an event from the subscribed calendar. To change anything you have to go to the original account.
It updates when the other calendar feels like it. The subscribing app decides how often, not you, and it can take hours; there's no reliable button to force it. Move the doctor's appointment in the morning and your work calendar may show the old time for a good part of the day, right when someone books over you.
It shows too much and counts too little. Google's secret address hands over your events with titles and details: anyone holding that link sees your whole calendar, and if it leaks you have to reset it, which breaks every subscription that used it. And the other way round: a subscribed calendar usually doesn't count as busy when a colleague looks for a slot with you, because it isn't your primary calendar.

When an ICS link is enough

There are cases where it's the right tool and you don't need to pay for anything:

Calendars you'll only look at: public holidays, the school calendar, your team's fixtures, the dates of a conference.
Giving someone you trust a view of your schedule, knowing they'll see it late.
A one-time move from one calendar to another, importing the file once.

If nobody books you based on that calendar and you don't mind changes arriving hours later, stick with the link.

When you need to sync instead of subscribing

Other people book you on one calendar and your commitments live on another.
You want to create, move or delete from either side.
You need a change to arrive in minutes, not hours.
You want to block the slot at work without showing the details.

A sync doesn't show one calendar inside another: it writes real events into the destination calendar and remembers which copy matches which original. That's why it can move or delete the copy when you change the original, and why, between Google and Microsoft 365, the copy sits on your primary calendar, where people looking for a slot with you can see it (what comes from iCloud lives in the separate "iCloud (Calen1)" calendar). If what you want is to bring work and personal together, this guide goes step by step: how to combine your work and personal calendars.

What Calen1 does differently from an ICS link

Calen1 connects to your accounts through each provider's official access: Google and Microsoft 365 with their own authorisation, and iCloud over CalDAV with an app-specific password. It doesn't read a link or depend on another app deciding when to refresh.

It checks roughly every two minutes, so a change shows up on the other side within a few minutes.
Two-way or one-way, pair by pair. On a two-way pair, what you create, move or delete in one shows up in the other, and vice versa. The step by step is in the guide to syncing Outlook with Google Calendar both ways.
Privacy mode: your Google events reach Microsoft 365 only as busy blocks, without the original title, the description or the location. Delete the original and it disappears from work; delete the busy block and the original isn't touched.
No duplicates on connect: if the same event was already on both calendars, it pairs it up instead of copying it again.

The Base plan costs $50 MXN a month or $500 a year, with a 5-day trial with a card. If you're coming from an ICS link, remove the subscription before connecting: otherwise you'll see every event twice, the subscribed one and the synced one.

Stop copying events by hand

Calen1 keeps Google Calendar, Microsoft 365 and iCloud in sync on their own, with no duplicates. From $50 MXN a month.

See plans

Frequently asked questions

Why doesn't my ICS-subscribed calendar update?

Because the subscribing app decides how often, not you. Google Calendar and Outlook re-read the link every so often, and that can take hours. It isn't something wrong with your setup: that's how subscriptions work.

Can I edit an event on a subscribed calendar?

No. A subscribed calendar is read-only: to move or delete an event you have to do it in the original account, and the change reaches the subscribed calendar when the link is read again.

Is it safe to share Google's secret iCal address?

With care. Anyone with that link sees your events in detail without signing in. If you think it leaked, reset it from the calendar's settings: the old link stops working and you'll have to re-subscribe the calendars that used it.

What's the difference between importing and subscribing to an .ics?

Importing copies the events once and whatever changes afterwards doesn't travel. Subscribing re-reads the link every so often, but the resulting calendar is read-only. Neither one is a two-way sync.

Can I use two crossed ICS links to get both directions?

You can, but you don't get a sync: you'll have two read-only calendars, each with its own delay, and you still won't be able to edit anything from the other side.

Does Calen1 use an ICS link to sync?

No. It connects with Google's and Microsoft 365's official authorisation, and over CalDAV with iCloud, and writes the changes straight into the other calendar; it checks roughly every two minutes.