Direction: one-way or two-way

A sync has one direction, and it applies to every calendar in it. Push is one-way. Mirror is two-way.

You choose it on the control between the two groups of calendars in the sync composer.

Push — one-way

Events copy from the source to the target. The source is never modified. Nothing is written back to it, ever, no matter what happens on the target side.

Use Push when:

• your work calendar should show your personal commitments as busy time, but your personal calendar shouldn't fill up with work meetings
• you're feeding several client calendars into one calendar you actually look at
• one side is a read-only feed you couldn't write to anyway

A Push can have several sources and several targets. Three client calendars feeding one working calendar is one sync with four calendars in it.

Mirror — two-way

Every calendar in the sync is both a source and a target. An event created in any of them shows up in all the others.

Use Mirror when both calendars are yours and you want either one to be a complete picture. Two work accounts after a job change; a personal Google account and an iCloud account you use on your phone.

What "two-way" doesn't mean

Mirror doesn't make the calendars identical. Each one gets a copy of the other's events, and copies still obey your appearance settings.

So if the title setting says the copy reads "Busy", a Mirror gives you real events on one side and blocks labelled "Busy" on the other, in both directions. That's working correctly, even though it doesn't look like a mirror in the everyday sense.

The same goes for filters. A Mirror with the RSVP filter set to Only-yes carries accepted meetings both ways and nothing else, in both directions.

Which one you actually want

The question that decides it: should events created on either calendar appear on the other, or only on one of them?

If the answer is "only one direction", Push is not a lesser choice — it's the correct one, and it's safer, because the source can never be touched.

A useful check: if you'd be alarmed to find a work meeting on your personal calendar, you want Push, not Mirror.

Changing direction later

You can. Going from Mirror to Push means the calendar that stops being a target no longer receives new copies, and the copies already sitting in it are removed.

You'll be shown a count of what's about to be deleted before it happens. Read that number. Nothing in SYNCDATE puts them back.

Why Mirror might be greyed out

Your selection includes an iCal or ICS feed subscription. Those are read-only — nothing can write to them — so a feed can only ever be the source of a Push. This isn't a plan limit and upgrading won't unlock it.

Mirror isn't refused outright, though: you can still switch to it, and doing so drops the read-only calendars out of the selection and tells you how many it removed. So the choice is really between a Push that includes the feed and a Mirror that doesn't.

If you want two-way sync with iCloud, connect it as a CalDAV account rather than subscribing to its ICS link.

Known limitations

• One direction per sync. You can't have three calendars where two mirror each other and the third only receives. That's two syncs, and Free allows one.
• A Mirror holds fewer calendars than a Push does. 2 on Free either way; 4 against 10 on Plus; 5 against 20 on Pro.
• Two syncs don't chain. Mirror A↔B and Mirror B↔C, and A and C never see each other — copies aren't passed on. Put all three in one Mirror instead. See What SYNCDATE can't do.
• The same pair can't be in two syncs going the same way. You'll be blocked when building the second one, which is deliberate: two syncs writing to one calendar is how duplicates happen.