Not sure if you are doing this for the new EmployeeHireDate attribute that's not available in AD DS.
I'm doing this by just putting text (String) into the msDS-extAttr1 and sync it to AAD and I just keep in mind that the format should be yyyyMMddHHmmss.fZ. The ".0Z" is important at the end.
Example: Set-ADUser SamAccountName -replace @{'msDS-cloudExtensionAttribute1' = '20501017082715.0Z' }
Keep in mind that the time is in UTC and should be set for Morning for the On-Boarding Workflow Lifecycle to work.