how to make the contents of a file disappear after x numbers of days or at date

[user1]

>> I should be a kind of wipe operation :-)
> Can you explain what you mean with "disappear"? Should the result be an
> empty file or e.g. a file with the same length and random data?


I mean the result should be an empty file (or a part of a file), where 
the "wiped" data is gone and cannot be restored.