Thank you for posting your query on Microsoft Q&A. From above description I could understand that you want to know when did the VM first onboarded to Microsoft Defender for Cloud.
Please do correct me if this is not the case by responding in the comments section.
Microsoft Defender for cloud is enabled at subscription level and plans available under Cloud Workload Protection (CWP) are responsible for actions on your resources.
For example if you are trying to target VM in your environment, then CSPM plan has to be enabled, and once enabled any VM deployed in the environment will be visible under defender for cloud inventory, this should be done on very same day when VM is deployed.
If you have enabled monitoring agent for the device you may get the details from Log-analytics
If you did push Endpoint protection say via Azure policy, then device would be visible then it would be visible on defender for endpoint portal along with onboarding date, which you could consider the time when device was on boarded to defender for endpoint.
Thanks,
Akshay Kaushik
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