SQL Server 2012 :: Return Only Count Of Database With Backup Older Than 24 Hours

I need only the count of databases that last fullbackup was older then 24 hours or null. and status is online. I have tried

SELECT Count(DISTINCT msdb.dbo.backupset.database_name)
From msdb.dbo.backupset
where datediff(day,backup_finish_date,GETDATE()) > 1 -- or is null
and Database_Name not in ('tempdb','ReportServerTempDB','AdventureWorksDW','AdventureWorks') --online also
group by Database_name, backup_finish_date

Tried using where max(backup_finish_date) < datediff(day,backup_finish_date,GETDATE()) .But get the aggregate in where clause error. get a count of databases with backups older than 24 hours not including the samples, report service, and tempdb. I would also want to put status is online but havent gotten the above to work so havent tried to add that yet.