My iCloud calendar has started behaving very strangely. Syncing issues, randomly changing date, but the weirdest thing is, if i login to iCloud from any browser, and enter an event it gets moved 3 days back, and always to 23:00. This behaviour only happens for events after 25th October 2019, and before March 30th. So all my distant future events are wrong, and even entering new ones online from an external browser, the event immediately jumps 3 days back. I have tried signing out of iCloud on all my devices, but it didn't seem to make any difference?
I've been through similar issues (not the same issue ... but iCloud weirdness that I couldn't pin to any device). Turns out it is possible to wedge iCloud even though all your local devices are ok.
Take backups of everything you can (your computer, phone, iPad, etc.)
Go to each device you own and log out of iCloud
It is critically important you log EVERY device out of iCloud because the fix relies on you having zero devices that are logged in.
- When NO devices are logged into iCloud, then (and only then) iCloud will dump it's cache for all data that it was syncing to your devices. If even a single device remains connected to iCloud ... this will not happen.
Pick your most reliable device and log back into iCloud. It will start syncing it's state (such as your calendar) to iCloud.
You can start re-reconnecting other devices back into iCloud one by one (I tend to do this very slowly to make sure each device has time to fully sync before I add the next device).
If the problem turns out to be corrupted iCloud cache (and this has happened to me), then this should fix it. If the problem is with a specific device then you may need further steps.
TL;DR: call 800-275-2273 for AppleCare; otherwise:
If adding events to the calendar in iCloud.com isn't working properly, it is much less likely that the issue is coming from your devices. Signing in and out on devices re-writes local files that maintain the calendar and deal with incoming or outgoing sync data. Just in case, have you restarted all your devices? To be clear, in iOS, restarting is using the slide to power off, not the hard reset. Slide to power off does maintenance on the device before it completely powers off. If that doesn't (didn't) help, try signing out of iCloud, restart the device, then sign back in.
Another test is to create a new calendar and see if it has the same issue. If not, you may just need to start using the new calendar instead.
You can test if the issue is coming from one of your devices by turning off Calendar in the iCloud preferences/settings on all your devices, then add an event on iCloud.com. If it doesn't work as expected, skip to the second to last paragraph below. If it works as expected, turn on the Calendar in iCloud preferences/settings for one device and try again. Repeat until it fails. Backup and restore the offending device back to factory and sign in with your Apple ID, but don't use your backup to restore the device. Once back on the home screen/desktop, go back to iCloud.com and try again. If it works, you have two choices: restore the device from backup to see if starts to fail again (if it fails again, the problem is in your backup and if it doesn't you're all set); or just use the device as new and rely on other methods to load your data again (starting, of course, with iCloud sync data).
Make sure you have a backup before making any other changes, however. And I would recommend both iCloud and iTunes. Having more than one backup is never a bad idea when making changes.
If still not resolved, I recommend calling Apple (1-800-275-2273). They always support Apple ID issues. They might need your input to help resolve the issue. It is likely you're not the only one, but it may be that the number of individuals with this issue is such a small number that every Apple ID used in troubleshooting and debugging will help. They may not be able to help resolve the issue on the first contact and, if my theory is right, they will likely escalate, perhaps twice.