Thank you for posting this in MIcrosoft Q&A.
You can find the date when the role was assigned only through user audit logs. Audit logs in Azure AD are present only for last one month if you have Azure AD premium license.
If you do not have the license then you can view only last 7 days logs.
Other that this we cannot find the date for Azure role assignment.
Let me know if you have any further queries.
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