I cannot find any way to export the registered date from AAD using any Powershell commandlets.
Unfortunately this is not possible as of now.
Device CreationTime is stored in deviceSystemMetadata and deviceKey attributes of the device, which are not exposed via Graph API version (api-version=1.6) used by PowerShell. This information is exposed via Graph API (api-version=1.61-internal), which is used by Azure Portal. This is why you can see the CreationTime on Azure Portal but not via PowerShell.
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You can look into Azure AD Audit logs and can track date of registration.
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