I am not sure whether there is such a registry key. At least Microsoft does not mention it.
However, you can manually control the dates displayed, five at a time.
Remember, Win98 only backs up registry when it was successfully booted.
Go to C:\Windows\Sysbckup and move all files rb0xx.cab, where xx is a number between 0 and 20, to a folder (let?s say c:\Windows\MyBackup).
Sort them by the date they were created not by the file name. Choose the five files according to the dates that you want and move them back to C:\Windows\Sysbckup folder.
Now you can restart the computer (use Restart in MS-DOS mode) and run ?Scanreg /restore?.
You may boot to Safe Mode Command Prompt Only or to Command Prompt Only to run the command. You may also boot the computer with Win98 Startup disk (you need to go to C:\Windows\Command).
If the five dates are not satisfactory, move them back to MyBackup folder, choose another five files, move them to C:\Windows\Sysbckup, and run the Scanreg again. You may repeat the process until you get what you want. This may not be exactly what you want but it gets the job done.
Reference: http://tinyurl.com/j9pe
http://tinyurl.com/fhv7