Which events enter a sync at all: the RSVP filter, working hours, Google's special event types and exclude rules — plus what tightening one deletes.
This group decides what enters a sync at all. Anything filtered out is never copied, and a copy that already exists is removed when a filter starts excluding it.
It's the second section in a sync's settings, headed 02 · Filters. Every setting here applies to the whole sync — you can't filter differently per calendar on any plan.
Three choices, based on your own reply to an invitation:
All is the default. If your calendar fills with invitations you never responded to, this is the setting that stops it.
Tightening this deletes copies. Switching from All to Only-yes removes the copies of everything you hadn't accepted. You'll see the count first.
Two rules that work together: which days of the week to sync, and one window of hours.
Set it to weekdays 09:00–17:00 and a Saturday event isn't copied, nor is a 20:00 one.
The hours are read in each event's own time zone, not yours. A 10:00 meeting created in New York is inside a 09:00–17:00 window even if you're in Berlin, where it's 16:00. There's no setting to change this. It matters if you travel or if colleagues elsewhere put things on a calendar you sync.
Three more things:
An event spanning several days is judged day by day. If any day it touches falls inside the window, it's kept.
An opt-in list, and this is the one that catches people out: on a sync you create today, nothing in it syncs unless you tick it.
Syncs that already existed when we added these filters are the exception. They were left with all four ticked on purpose, so they'd keep behaving the way their owners expected, and they still sync all four. If you're not sure which yours is, the ticks on the sync itself are the answer.
Four types, all of them things Google Workspace creates and marks as not-quite-events:
Ordinary events are never affected by this. It only governs these four.
If you want your Out of Office to appear on your other calendar, tick Out of Office here. If your other calendar keeps filling with birthdays, leave that one unticked — which it is by default.
This only works on Google. Outlook, iCloud and other CalDAV accounts don't mark these types at all, so an Outlook Out of Office is an ordinary event to us. The checkboxes do nothing for those accounts in either direction — they can't leave an Outlook out-of-office block out, and they can't bring one in. To keep one out, use a rule that matches its shape instead: see What SYNCDATE can't do.
A list of rules you add. Three kinds:
You can add several "Title contains" rules. You can only add "All-day events" and "Free events" once each.
Title contains matches anywhere in the title, not just the start. A rule for "Gym" also skips "Gym class rescheduled".
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Still stuck? Support will read your actual setup and tell you what it sees — describe what you expected and what you got.