Google Calendar is more tolerant than Outlook or Apple Calendar, but it still rejects updates in specific situations. Developers often assume Google always applies updates because Google is forgiving with malformed ICS files, but Google has clear rules that determine whether an update is accepted, ignored, or treated as a new event.

If your update looks correct but Google does not change the event in the user's calendar, the problem is almost always related to one of three things: the update was considered stale, the event identity changed, or Google normalised something you were relying on.

This guide explains why that happens and how to generate ICS updates that Google will reliably apply.

Google is tolerant, but not naive

Developers get misled by how forgiving Google is with initial event creation. Google will happily accept:

• missing SEQUENCE fields
• malformed RRULE syntax
• slightly broken VTIMEZONE entries
• missing PRODID
• nonstandard formatting

Because of this, developers assume updates will be handled the same way. They will not.

Google only tolerates malformed ICS during creation. When it comes to updates, Google applies stricter rules because it treats calendar events as server-side canonical records.

Once Google has stored an event, any incoming update must precisely match or intentionally supersede the stored version. If it does not meet Google’s internal criteria for identity and recency, Google throws it away silently.

Why Google ignores updates that look correct

Google decides whether to apply an update using three fields: UID, SEQUENCE, and DTSTAMP. The behaviour overlaps with Outlook, but Google adds its own logic on top.

The three update killers are:

The UID does not match exactly

If your system regenerates the UID or alters it even slightly, Google treats the update as a new event. It might show a duplicate, or it might ignore the ICS completely.

The SEQUENCE number is lower than or equal to the stored event

Google treats SEQUENCE as a version gate. If SEQUENCE has not increased since the previous version, Google assumes the ICS is stale and discards it.

Google is more forgiving than Outlook and may apply updates without SEQUENCE, but as soon as SEQUENCE is present, Google enforces it.

The DTSTAMP is older than the stored DTSTAMP

This is the unique gotcha with Google. Even if SEQUENCE is correct, Google will reject an update if DTSTAMP is older than the version it already stored.

This is why updates sometimes work in Outlook but do not propagate in Google.

If your build pipeline or server clock regresses by seconds, or you reuse an earlier DTSTAMP without realising, Google considers the update invalid.

A real example of an update Google will reject

BEGIN:VCALENDAR
VERSION:2.0
METHOD:REQUEST
BEGIN:VEVENT
UID:weekly-sync-777@example.com
DTSTAMP:20250110T090000Z
SEQUENCE:2
DTSTART:20250115T100000Z
DTEND:20250115T110000Z
SUMMARY:Weekly Sync
ORGANIZER:mailto:team@example.com
END:VEVENT
END:VCALENDAR

This update will be ignored by Google if the stored event has a DTSTAMP later than 20250110T090000Z. Even though SEQUENCE is correct, the event would still be considered stale.

A corrected update Google will accept

BEGIN:VCALENDAR
VERSION:2.0
METHOD:REQUEST
BEGIN:VEVENT
UID:weekly-sync-777@example.com
DTSTAMP:20250110T120000Z
SEQUENCE:3
DTSTART:20250115T093000Z
DTEND:20250115T103000Z
SUMMARY:Weekly Sync (Updated Time)
ORGANIZER:mailto:team@example.com
END:VEVENT
END:VCALENDAR

Google sees:

• identical UID
• SEQUENCE increased
• DTSTAMP newer than stored version

This combination guarantees that the update is recognised and applied.

Why Google sometimes overwrites your timezone

One of Google’s core normalisation behaviours is converting ambiguous or unknown timezones into UTC. This does not usually block updates, but it can create confusion:

• you send an update in Europe/London
• the user sees a UTC event
• your next update uses Europe/London again
• Google compares your updated DTSTART to the stored UTC version
• Google determines nothing changed

So the visual update does not show, even though your ICS file is different.

This is a subtle but common case, especially around DST boundaries. Google internally picks the canonical interpretation and refuses to update the event unless the server-side representation actually changes.

Why recurring event updates fail in Google Calendar

Google splits recurring events into multiple related events internally. When you update a recurring VEVENT, Google checks whether the update applies to:

• the master recurring rule
• one specific instance
• an exception instance
• a previously split fragment

If your update does not target exactly the record Google expects, it may apply to one fragment but not another. Symptoms include:

• only one attendee sees the update
• only one instance updates
• the series appears unchanged
• the first or last instance shifts unexpectedly

This is an inherent limitation of ICS combined with Google’s internal model.

Why Google silently discards METHOD mistakes

Many developers accidentally send updates with:

METHOD:PUBLISH

Google interprets METHOD:PUBLISH as "informational event data" rather than a meeting update. Google does not notify attendees, does not treat it as an update, and does not change the event.

Likewise, cancellations must use:

METHOD:CANCEL

A cancellation using METHOD:REQUEST is ignored.

Google does not warn you when this happens.

How to generate updates Google will always honour

Store three fields server-side

For every event, store and maintain:

• UID
• SEQUENCE
• last DTSTAMP

Never regenerate these values on the fly.

Increment SEQUENCE once per intentional update

Do not tie SEQUENCE to timestamps.
Do not reset it.
Do not calculate it from hashes.
Increment it like a version number.

Ensure DTSTAMP is monotonic

If your server ever emits a DTSTAMP older than the stored version, Google may reject the update.

This is especially important in:

• multi-region systems
• CI pipelines
• systems that rebuild events from templates
• any situation where multiple writes happen in parallel

Keep ORGANIZER constant

Changing the organiser is treated as a different event identity.

Avoid relying on Google to preserve your timezone choice

Send absolute times consistently.

Treat timezone reconciliation as a vendor-specific behaviour, not a guarantee.

How Synara solves the Google update inconsistency problem

Synara removes these concerns by treating your event as a canonical JSCalendar object. Synara maintains:

• SEQUENCE incrementing
• DTSTAMP monotonicity
• UID stability
• vendor-specific ICS formatting
• METHOD correctness
• timezone reconciliation rules

When Synara sends an update to Google, it already satisfies the constraints that cause Google to reject updates from most custom ICS generators.

Instead of fighting Google’s internal rules, Synara works with them.

Summary

Google Calendar ignores ICS updates when:

• the UID changes
• the SEQUENCE number does not increase
• DTSTAMP moves backwards
• the client normalises the timezone in a way that hides your changes
• METHOD is incorrect
• recurring updates do not match Google’s internal event fragments

Google is liberal when creating events but strict when applying updates.

Once you understand the rules Google enforces, updates become predictable.

Further reading inside this series

• Why Outlook Ignores ICS Event Updates

If you want deterministic updates across all calendar vendors

Synara gives you deterministic calendar behaviour across Google, Outlook, and Apple without maintaining ICS logic yourself. You send a clean JSCalendar object and Synara handles UID and SEQUENCE management, vendor-specific ICS formatting, timezones and DST rules, update propagation, RSVP behaviour and recurring event consistency.

If you're tired of debugging differences between calendar clients, Synara ensures every attendee sees the same event regardless of their calendar.

Learn more → https://synara.events
API docs → https://synara.events/docs