recording last access to file in mysql db

Hi,
I have a download site written in php, currently it doesn't record last access to a file in my db , but I want to add this feature , so I can remove the files which are not accessed for a long time.
first I need to know what type of data should be the field I add to mysql table , timestamp or timedate or int ?
also I need to update this field whenever a user downloads a file , I just need the mysql update statement , I know where to put it in my code :D
lastly I need to know how may I compare the last access time recorded in my db with current time , so I can delete old files.
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