Sync calendars across Microsoft 365 tenants

You work in two Microsoft 365 tenants, for example as a founder of two companies or as a consultant with a client account. Free/busy sharing between tenants needs an admin on both sides and often does not exist. This guide syncs the calendars yourself, as busy blocks.

Steps

1. Create a Microsoft Entra app with the account type Accounts in any organizational directory and personal Microsoft accounts. A single-tenant app cannot connect the second tenant.
2. Deploy slotmirror with the Quickstart and set PUBLIC_URL.
3. Connect one account per tenant under Accounts. If a tenant blocks user consent, its admin must grant consent to the app once.
4. In Where your events go, add the calendars to the group Block each other.
5. Choose Save.

Names in the editor

slotmirror derives a short name from each address. You can rename an account, for example to the name of the company, so that the overview reads “from Company A to Company B”.

Things to know

• Each tenant sees only “Busy”, marked private, unless you choose more detail per direction.
• If you sign in with one of the tenants and want only that tenant to sign in, see Organisations.