SCCM 2012 query machines not contacting from 15 Days

You can apply this query on SCCM 2007/2012. This query will show the machines that are not contacting with SCCM from the last 15 days. 

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select 
SMS_R_System.ResourceID,
SMS_R_System.ResourceType,
SMS_R_System.Name,
SMS_R_System.SMSUniqueIdentifier,
SMS_R_System.ResourceDomainORWorkgroup,
SMS_R_System.Client from SMS_R_System where AgentName in ("Heartbeat Discovery") and DATEDIFF(day,AgentTime,GetDate())>=15

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