Follow the below steps to find out the particular modified file in between 20 to 30 days & delete the same.
Command to find and list the file
#find / -mtime +20 -mtime -30 -type f -name test.* -exec ls -al {} \;
Command to delete the listed file.
#find / -mtime +20 -mtime -30 -type f -name test.* -exec rm {} \;
As per your needs, change the file name.
Hope it helps.