Integration Services :: Expression To Parse Filename To A Date

I am very new to SSIS.  I need a package to iterate through files in folders and subfolders, evaluate the date of the file and delete them if they are over 90 days. I am using the following as a guide.  The reason I am not using a maintenance plan for this is that I did not see a way to use a built in maintenance plan task to copy files.  I already built a package that copies files to the destination.  Now I need to delete files in the destination that are over 90 days old.  

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I have a File System Task to delete files. I have a Script Task with a precedence constraint where I need to build an expression on the constraint between the Script Task and the File System Task for the delete.  

I have database backups in the format of SQLInstanceName_DatabaseName_BackupType_YYYYMMDD_XXXXXXX.bak.  

I created a variable to store the filename and I need an expression that will convert a file name like :

MySQLInstance_MyDB_DIFF_20150803_1700000.bak into 20150803, compare it to getdate -90 days.  

I created another variable for the Max File Age.  I built the below but when I click on Evaluate Expression I get an error stating, "The expression might contain an invalid token, an incomplete token, or an invalid element." 

DATEDIFF("dd",(DT_DATE)(SUBSTRING(RIGHT( @[User::strFileName], 20 ),1,8), GETDATE()) >  @[User::intFileMaxAge]