Why Google Calendar Sometimes Ignores ICS Updates (and What Developers Can Do About It)

If you have ever sent calendar invites from your own backend, you have probably seen this problem:

• You send an event update

• The ICS file validates

• Google Calendar imports it without errors

• Nothing changes

No error. No warning. The update simply does not apply.

This is one of the most common issues developers face when working with ICS. The root cause is that Google interprets the specification differently from Outlook and Apple Calendar. The behaviour is not random. It is the result of several strict internal checks.

Below is a clear explanation of what actually happens, why it happens, and what you can do to avoid the silent failures.

1. Google applies updates only when several conditions match perfectly

Developers usually focus on UID and SEQUENCE. Google checks more than that. An update will only apply if all of the following match Google’s internal model:

• The UID matches an existing event

• The SEQUENCE value is higher

• The VTIMEZONE block matches the original event exactly

• DTSTART and DTEND changes pass Google’s normalisation rules

• STATUS, ATTENDEE fields and alarms do not conflict with the stored event

If any of these checks fail, Google usually accepts the file but ignores the update without notifying anyone.

2. Outlook and Apple Calendar behave differently

The inconsistency is the real source of confusion. For the exact same ICS update:

• Outlook may apply it correctly

• Apple Calendar may apply it but display it slightly wrong

• Google may ignore it without any message

Each client uses its own logic. This means a file that looks valid and behaves correctly in Outlook may not work in Google Calendar, even though both clients claim to support the same specification.

3. Timezone mismatches are the most common silent failure

This is the problem that catches the most developers.

If the timezone definition in the update file is not identical to the one stored with the original event, Google often refuses to apply the update.
The definitions must match character for character. Even a small difference can cause the update to be dropped.

This behaviour is allowed by the spec, which is why it is so difficult to reason about.

4. Ways to reduce the chance of silent update failures

These steps are practical and consistently improve results:

Always increment SEQUENCE
Even if the change is only a description update.

Keep VTIMEZONE definitions consistent
Use the same timezone block for the original event and all updates.

Send full VEVENTs, not partial updates
Google, Apple and Outlook interpret partial VEVENTs differently.

Test each update flow against all three major clients
Validation is not enough. Actual behaviour varies by client, so real testing is required.

5. If you need reliable behaviour across Google, Outlook and Apple

You can manually maintain:

• Vendor specific logic

• Timezone normalisation rules

• Multiple schemas for ICS, Google API and Microsoft Graph

• Workarounds for SEQUENCE and RECURRENCE edge cases

• Testing infrastructure for every update scenario

Many teams try this and eventually realise it becomes its own infrastructure project.

If you want to avoid building that system yourself, Synara handles the cross vendor differences automatically: https://synara.events

Synara validates and transforms events so that updates apply consistently across all major calendar clients. It removes the guesswork and prevents the silent failures that are common when using plain ICS files.

Final thought

ICS is a legacy specification with wide areas of ambiguity. Google, Outlook and Apple each fill in those gaps differently. The result is an ecosystem where updates can succeed, partially apply or be ignored with no feedback.

Understanding these differences is essential for building a reliable invite workflow. This is also why tools like Synara are becoming necessary for modern applications that rely on calendar features.

If you want a similar article about Outlook duplication issues or about Apple Calendar not surfacing updates, I can write those next.