You can filter in any report on the field tblAssets.Lastseen. E.g. the report below or you can create a dynamic report by filtering on tblAssets.Lastseen > getdate() - 30 to always filter on the assets successfully scanned in the past 30 days.
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tblAssets.IPAddress,
tsysAssetTypes.AssetTypename As [Asset Type],
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.Scanserver,
tblAssets.Firstseen,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblAssets.Lasttriggered,
tblAssets.LastScheduled,
tblAssets.LastActiveScan,
tblAssets.LastIPScan,
tblAssets.LastLsPush,
tblAssets.ServiceVersion,
tblAssets.LsPushVersion
From tblAssets
Inner Join tsysAssetTypes On tblAssets.Assettype = tsysAssetTypes.AssetType
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Where tblAssets.Lastseen >= '10 / 01 / 2013'
Order By tblAssets.AssetName Desc