Hi Frank Fowlie,
Welcome to Microsoft Q&A forum.
To better narrow this down, could you please confirm a few details?
- Are you unable to change the date/time for new iCloud calendar events, existing events, or both?
- When you try to change the date/time, are the fields greyed out, or do they revert back automatically?
- Does the same calendar item allow editing correctly from iCloud.com or your iPhone/iPad Calendar app?
- What version of iCloud for Windows are you using?
In the meantime, please try opening the event, selecting Scheduling Assistant, then switching back to Appointment and checking whether the date/time can be edited there.
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