Security and privacy

Hello, I am an administrator of my Moodle sites and I want to change the date of one sent file into the system through the activity assignment. The file was sent by a student. I have the access to MySQL by phpMyAdmin and I know the name of the file and the name of the student, who sent his file. In phpMyAdmin I found the name of this file in table "files" and I changed the value in the field "timecreated" and "timemodified" (I reduced it about 15.000.000 seconds) in the record about this file but the date in the browser stayed identical. What should I do to change the date of the file about half a year back? Thank you for your answer. Ivan K. Slovakia