Convert datetime into hours

Hi,

Below query returns if a backup hadn't occured in last 7 days. But my reuirement is to check for last 24 hours backup status. Please let me know, how to convert below query to check last 24 hours or 12 hours  or 6 hours back backup status.

SELECT  (SELECT   RTRIM(CONVERT(char(20), SERVERPROPERTY('machinename'))) + '_' + CONVERT(char(20), SERVERPROPERTY('instancename'))) as instance,
        sd.name as name,
        bs.type,
        bs.database_name,
        max(bs.backup_start_date) as last_backup,
        note = CASE
   WHEN max(bs.backup_start_date) < GETDATE() - 7 THEN 'ALERT'
   WHEN ISNULL(max(bs.backup_start_date),0) = 0 THEN 'ALERT'
   ELSE '---'
   END
FROM    master..sysdatabases sd
        Left outer join msdb..backupset bs on rtrim(bs.database_name) = rtrim(sd.name)
        left outer JOIN msdb..backupmediafamily bmf ON bs.media_set_id = bmf.media_set_id
        WHERE sd.name NOT LIKE '%tempdb%'
Group by sd.name,
        bs.type,
        bs.database_name
Order by sd.name,last_backup


Thanks in Advance.