Hi Navar,
Working Set - Changed Today will actually look for the files with their dates. All the files created or modified today and has today's date, will be backed up.
Please refer below mentioned articles.
About the working set backup method
http://www.symantec.com/docs/HOWTO22706
What are the advantages of using the Backup Exec for NetWare's Working Set Backup. Why use it instead of Incremental or Differential backups?
http://www.symantec.com/docs/TECH4159
Also to let you know, this Working set backup method has been removed from any version above BE 2010R3.
So, in BE versions above 2010 R3 you can go for the workaround mentioned below.
Go to the selection pane and click EDIT and then right-click on the server. You will have an option to include/exclude files. Click on it and choose include files accessed for X number of days.
Thanks!!