Calendar sync without connecting an account

"Sync" usually means "hand over access"

Every multi-calendar tool starts the same way: a Connect button, a Google or Microsoft consent screen, an Allow click. Calendar sync without connecting an account sounds like a contradiction, because the whole category was built on account connections. manyCalendars is the exception. It never asks you to connect anything. It reads the calendar tabs you are already signed into, right in your own browser, and merges them into one view.

That one design choice changes the security story, the privacy story, and, for a lot of consultants and contractors, whether the tool can work at all. Here is what it actually means.

How Tab Sync works

The feature is called Tab Sync. You open your calendar the way you already do: outlook.office.com for a client's Microsoft 365 account, calendar.google.com for your own. In the manyCalendars extension you tell that calendar to sync, and from then on the calendar follows its tab. Whatever events are visible in the tab appear in your combined view, color coded next to every other calendar you track.

Tab Sync is read-only on the source. It never clicks anything, never edits an event, never types into the page. It reads what your browser is already showing you, because you are signed in, and stores it locally in the extension. When the tab refreshes, your view refreshes.

What "no account connection" buys you

• No token to steal. OAuth-based tools hold a standing credential to your calendar on their servers. If that vendor is breached, the token and the cached data go with it. manyCalendars holds nothing. We wrote up the full risk picture in the hidden risks of OAuth calendar access.
• No server copy. Your events never leave your machine. There is no manyCalendars cloud with your client roster on it. A breach of our website could not expose your calendar, because our website never sees it.
• No scope creep. An OAuth grant like Calendars.Read covers every calendar in the account. Tab Sync can only read what you can see in the tab in front of you. Nothing more is technically possible.
• Trivial revocation. Sign out of the tab, or remove the extension. Done. No admin console, no token audit, no support ticket.

Why this matters more since November 2025

On managed Microsoft 365 tenants, connecting an account is often no longer your call. Since November 2025, Microsoft Entra default settings block users from consenting to third-party apps that request calendar permissions (Calendars.Read or Calendars.ReadWrite). An admin has to approve every app, and only existing grants were grandfathered in. If you work across several client tenants, that means an admin-approval request at each one, filed by you, an outside contractor, into a queue nobody owns. We cover the change in Microsoft blocks calendar apps by default.

Tab Sync sidesteps the whole question because there is nothing to consent to. You are not granting an app access to the account. You are looking at a page you already have open. As far as we know, it is the only approach that works when IT says no. If that is exactly your situation, start with the full guide to getting a combined calendar without admin approval, or the shorter how to see the calendars IT blocked.

The honest limits

manyCalendars is a desktop browser extension for Chrome, Edge, and Firefox. There is no mobile app.

Tab Sync reads what is visible in a tab you are signed into. Sign out and that calendar stops updating until you sign back in. Your view keeps the last events it read, so it does not go blank in the meantime.

And it is read-only. If you need true two-way sync, where an event created in one account is written into another, that requires server-side access, which means OAuth and, on most managed tenants, admin approval. If your admins will approve it, CalendarBridge does that job well: patented server-mediated two-way sync, booking pages, and a HIPAA BAA, at $4, $8, or $32 per user per month depending on plan. See our comparison, manyCalendars vs CalendarBridge, and the deeper look at merging calendars without OAuth.

What it costs

The Free tier is $0, permanently: unlimited manual calendars from .ics files and feed URLs, month, week, and day views, color coding, 100 percent local, no account needed.

Tab Sync is part of Pro at $15 a month, or $150 a year, which works out to $12.50 a month. Pro also adds meeting reminders, a custom work-week view, time zones, and .ics export. Full tier details are on the pricing page.

Every install starts a 14-day full trial with everything unlocked. No card. And, fittingly, no account. Install it free and sync your first tab in about a minute.

Frequently asked questions

Does manyCalendars ever see my password?

No. You sign in to your calendar the way you always do. The extension never asks for, handles, or stores credentials. It reads the events your signed-in tab is already displaying.

Does my calendar data leave my computer?

No. Events are stored locally by the extension in your browser. There is no manyCalendars server holding your data, which is also why there is no account to create.

Is this two-way sync?

No. Tab Sync is read-only on the source. You make changes in the source calendar itself, and your combined view picks them up when the tab refreshes.

Which calendars does it work with?

Tab Sync works with Google Calendar and Outlook on the web. The Free tier also takes unlimited manual calendars from .ics files and feed URLs, from any calendar system that provides them.