How is Backup Age calculated for MS SQL DB monitor?

How is Backup Age calculated for MS SQL DB monitor?

Backup Age displays the number of hours passed since the backup operation was completed. In simple terms, it is the difference between the current time and the time at which backup was performed.
Backup Age (hours) = Current Time - Backup Time
By default, we collect the details of the backup taken in the last 7 days. This can be changed by specifying the number of days for the textbox present in 'Collect SQL Database Backup Details' option under Admin -> Performance Polling -> MS SQL -> Metric Name -> Backup.
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For example, the following query is used to calculate Backup Details (under Backup/Restore tab) for the last 100 days (specified using the above option) in MS SQL DB monitor:
SELECT server_name, BS.database_name, backup_start_date, backup_finish_date, expiration_date, BS.recovery_model, is_damaged,CAST((CAST(DATEDIFF(s,backup_start_date,backup_finish_date) AS int))/3600 AS varchar) + ' hours, ' + CAST((CAST(DATEDIFF(s,backup_start_date,backup_finish_date) AS int))/60 AS varchar)+ ' minutes, '+ CAST((CAST(DATEDIFF(s,backup_start_date,backup_finish_date) AS int))%60 AS varchar)+ ' seconds' AS 'TotalTime', BS.type AS backup_type, backup_size, logical_device_name, physical_device_name, BS.name AS backupset_name,description,DATEDIFF(hh, backup_finish_date,GETDATE()) AS Backup_Age FROM  (SELECT M.database_name,M.type,max(M.backup_finish_date) AS maxtime FROM msdb.dbo.backupset AS M GROUP BY M.database_name,M.type) AS maxvalue ,msdb.dbo.backupset BS INNER JOIN msdb.dbo.backupmediafamily ON BS.media_set_id = msdb.dbo.backupmediafamily.media_set_id INNER JOIN sys.databases db ON db.name=BS.database_name WHERE (CONVERT(datetime, BS.backup_start_date, 102) >= GETDATE() -100) AND backup_finish_date=maxvalue.maxtime AND BS.type=maxvalue.type AND BS.database_name=maxvalue.database_name ORDER BY backup_finish_date desc

Note:
  1. In addition to the above option, there is an option Delete backup entries from Applications Manager when its database is removed, enabling which will delete the backup entries when its corresponding database is deleted from its SQL Server instance. 
  2. If enabled, it will delete the backup entries when the backup is not taken for more than the number of days mentioned in the Store SQL Database Backup Details option.
  3. By default, it will delete the backup entries when the backup is not taken for more than 7 days.