Summary
Outlook calendar not syncing has two different fixes, and one check tells you which you need: open the same calendar in Outlook on the web. If the missing event is there, the server is right and your local copy is stale — in classic Outlook, right-click the calendar folder, select Properties, then the General tab, then Clear Offline Items, and then Send/Receive > Update Folder. If the event is missing on the web too, it never reached that account, and when it actually lives on a personal Google calendar, an iCloud calendar, or a second Microsoft 365 tenant, no Outlook setting brings it across.
The split matters most to anyone whose week is spread over two accounts. Priya reads a work Microsoft 365 calendar in Outlook and keeps her evenings in a personal Google calendar; the Thursday 2pm dentist is real, Outlook shows that slot free, and a colleague books over it.
The weakest link is whatever holds Outlook's permission to see the other account. Microsoft removed connected accounts from Outlook.com outright, a subscribed link is read-only and refreshes on Microsoft's schedule rather than yours, and Microsoft has documented Gmail accounts in classic Outlook that stopped syncing without ever raising a sign-in prompt.
We built Keeper for the second case: real events written into your Outlook calendar from an account Outlook cannot sign into itself.
Native routes come first below, including the one that genuinely works, then the copy.
Two neighbouring jobs are elsewhere. Google events that stopped showing on an iPhone are Google Calendar not syncing with iPhone, and Apple Calendar failing against iCloud itself is Apple Calendar not syncing. Copies running in both directions between Google and Outlook are how to sync Google Calendar with Outlook.
The gap between those two views is the one on Microsoft Q&A: "I added an event to my outlook calendar yesterday morning. In the afternoon all calendar events for the year had disappeared." A day of view resets, cache clears and profile repairs later, the poster reports the events still sitting in the calendar that feeds this one — "All of the events are still visible on the second calendar but is not updating now."
Clear Offline Items, then Update Folder
The controls are Properties > General > Clear Offline Items on the calendar folder, and Send/Receive > Update Folder on the ribbon. This is the route for classic Outlook for Windows, and it applies when the event is already correct in Outlook on the web.
Microsoft, Synchronization problems in Outlook and OWA (checked 19 August 2026): "If there is an issue with a specific folder, you can resync the folder in Outlook." The steps on that page are: "Right-click the folder, and then select Properties", "Select Clear Offline Items, and then select OK", "On the Outlook 2010 ribbon, select the Send/Receive tab", "Select Update Folder."
The same page names the cause it is treating and the next step if the resync does not take: "When these symptoms occur, or you have other problems synchronizing, the problem might be a corrupted .ost file." And: "If these methods do not resolve the issue, we recommend that you rebuild the .ost file."
That local copy exists because of Cached Exchange Mode. Microsoft, Turn on Cached Exchange Mode (checked 19 August 2026): "In this mode, a copy of your mailbox is saved on your computer. This copy provides quick access to your data, and it is frequently updated with the server that runs Microsoft Exchange." Microsoft does not publish an interval for "frequently updated" on that page. Two boundaries it does publish: "Cached Exchange mode is not available in new Outlook for Windows", and "You cannot use Cached Exchange Mode with a POP or IMAP account."
There is one case where the stale local copy is expected rather than broken. Microsoft, Synchronization fails after mailbox items changed (checked 19 August 2026): "When you close your Outlook client and then reopen it later, changes that were made to your mailbox items while Outlook was closed aren't synchronized as expected." That page's verdict on it: "Under these conditions, the synchronization failure is expected for Outlook in cached mode." Its workaround is the same Clear Offline Items, or using Outlook in online mode.
This is the working fix when one Outlook account is not updating itself. It moves nothing between accounts, because there is only one mailbox involved.
Subscribe from web
The control is Add calendar > Subscribe from web in Outlook.com and Outlook on the web, where you paste the other calendar's iCal URL and select Import.
Microsoft, Import or subscribe to a calendar in Outlook.com or Outlook on the web (checked 19 August 2026): "When you subscribe to a calendar, your calendar will automatically refresh if the other calendar is updated. This can sometimes take more than 24 hours." The same page prints the cadence for a personal account: "This update can take more than 24 hours, although updates should happen approximately every 3 hours."
The Upload from file option on that panel is worse for this job, and Microsoft says so plainly: "When you import an .ics file, your calendar doesn't refresh the imported events, even if the calendar's owner makes an update." And: "When you import an .ics file, you get a snapshot of the events in the calendar at the time of import."
Neither route sends anything back the other way. Microsoft describes subscribing as the owner's changes reaching you: "Whenever the owner of a calendar you've subscribed to makes changes to the events, Outlook.com updates your calendar."
A subscribed link is a read-only view on Microsoft's own refresh schedule, so an edit you make in Outlook never reaches the source calendar.
Connected accounts in Outlook.com
There is no control any more. Outlook.com once let you attach a Gmail or Yahoo account to the mailbox itself, and that is worth ruling out before you go hunting through settings for it.
Microsoft, Connected email accounts like Gmail and Yahoo not working in Outlook.com (checked 19 August 2026): "Microsoft removed the ability to connect new third-party accounts such as a Gmail account and sync existing connected accounts from within Outlook.com."
The alternatives Microsoft lists on that page are the desktop and mobile Outlook apps, or mail forwarding: "you may still be able to add the account to other Outlook versions, such as Outlook mobile, the desktop versions of Outlook for Windows or Mac, or by forwarding those emails to another Outlook.com account." Forwarding moves mail. It does not move events.
Outlook.com in a browser has no third-party account route left at all, and existing connections were stopped, not grandfathered.
Sync the account to the Microsoft Cloud
The control is Add account in new Outlook for Windows, new Outlook for Mac, or Outlook for iOS and Android — signing in to the other provider rather than pasting a link. This is the closest thing to a native answer, and the limits are per client and per provider.
Microsoft, Sync your account in Outlook to the Microsoft Cloud (checked 19 August 2026): "This is available for Gmail, Yahoo, iCloud, and IMAP accounts in Outlook for iOS, Outlook for Android, and new Outlook for Mac. This is also available for Gmail and Yahoo accounts in new Outlook for Windows."
Read that list twice if you are on Windows. iCloud is on it for iOS, Android and Mac, and not for new Outlook for Windows. Classic Outlook for Windows is not on the list at all.
It is also a copy held somewhere new. Same page: "Syncing your account to the Microsoft Cloud means that a copy of your email, calendar, and contacts will be synchronized between your email provider and Microsoft data centers."
The calendar caveats have their own page. Microsoft, Known issues syncing Google accounts to the Microsoft Cloud (checked 19 August 2026): "The Open Shared Calendar button doesn't work with Google calendars since shared calendars automatically appear. Calendar shared with Free & Busy permissions doesn't work." Then: "There is currently a 20-calendar limit for calendars shared with you." And a privacy default that runs the opposite way to Keeper's: "If you create an event on your calendar in Outlook, its default visibility is 'Public,' regardless of your Google Calendar settings."
The grant underneath it can also lapse quietly. Microsoft, Users get errors 0x800CCC0E & 0x800CCC0F synchronizing Gmail and Yahoo accounts in classic Outlook (checked 19 August 2026): "Some Gmail and Yahoo accounts stopped syncing as of February 26, 2026. Accounts could also stop syncing after a password change. In both cases when Outlook stops syncing you are no longer prompted to sign in to your account."
This works for a Google or Yahoo account in a current Outlook app, and it is the honest native answer there. It has nothing for iCloud on Windows, nothing for classic Outlook, and it does not put those events on the work calendar your colleagues actually book against.
Outlook mobile and Exchange ActiveSync
The controls are the calendar list behind the three lines in Outlook mobile, and Settings > the account > Reset Account. This is the phone case, where the calendar is fine on the desktop and empty in your pocket.
Microsoft, Can't sync calendar and contacts with my phone or tablet (checked 19 August 2026) starts with the protocol, because it decides whether a calendar can arrive at all: "IMAP and POP are sync protocols that are unable to sync calendar, contacts, etc. They can only sync email." And: "You must have an email provider that uses Microsoft Exchange ActiveSync."
The check is in the app: "Under your email accounts, make sure 'Calendar' is selected. If your email address is not displayed in the list, it's not set up using Exchange ActiveSync. You'll have to delete and re-add your account and make sure not to use IMAP."
If it is set up correctly and still empty, the page's next step is Reset Account: "In the Outlook app, go to Settings> Tap the account that is not syncing > Tap Reset Account."
The same page draws the line this whole post turns on, for events that were created on the phone itself: "In order to have your device's calendar events display in your Outlook calendar, they must be added through an account that has been synced to Outlook."
Reset Account fixes the phone. An event that never reached an account Outlook is signed into stays invisible on every device you own.
Copy the other calendar into Outlook
Only for the second case. If the event is already correct in Outlook on the web, go back up to Clear Offline Items — a copy will not make a stale local cache any fresher.
- Sign in at Keeper and connect Outlook. Microsoft's sign-in asks to see your calendars and to add or remove events, which is what writing a copy requires.
- Connect the account the events actually live on: Google, iCloud with an Apple app-specific password, Fastmail, a CalDAV server, or a second Outlook or Microsoft 365 account. Linked accounts and calendars covers what each one asks for.
- Open the source calendar and, under Send Events to Calendars, tick your Outlook calendar. That is one connection, and it runs one way. Sending events to another calendar has the detail.
- For the reverse direction, open the Outlook calendar and build the second one-way connection back. Two-way is two one-way connections, and Free covers 2 linked accounts and 3 connections.
- Leave the privacy defaults alone unless you mean to change them. A copy carries a placeholder title taken from the source calendar, with the description and location hidden; turning either on is a Pro setting per calendar. Guest lists are never copied, so nobody on the original invitation hears from your work tenant. What a copied event shows lists the fields.
- Then look at the Outlook calendar. Sources are read every minute on every plan. A Free destination is written every 30 minutes; a Pro destination is written every minute, and on Pro both Google and Outlook notify Keeper when something changes. If nothing has landed, an event has not arrived is the checklist.
Pro is $5/month or $45/year. A work Outlook calendar alongside a personal one is sync work and personal calendars.
Why we built Keeper for this
Every native route above depends on Outlook holding a live grant to the other account, on whichever client you happen to be using. Keeper holds its own grant to each account you connect, so a copy does not stop because Outlook's Gmail token lapsed, and it does not care that iCloud is missing from Microsoft's Windows list.
The grants are even-handed about it. Revoke Keeper's access at Google, or change the Apple Account password behind an app-specific password, and Keeper goes quiet too and asks you to sign in again — the same as Outlook would.
What lands in Outlook is an ordinary event on a calendar you own, not a subscription waiting on somebody's refresh cadence. Colleagues booking your week see the slot as taken.
Keeper cannot repair a mailbox that is out of date against its own server — that is still Clear Offline Items and Update Folder. What it removes is the assumption that Outlook can see every calendar your week is actually in.