Warning! Brian does point out that this technique may be no good if the file name needs to be known at a later stage in the package becuase the time will have changed (eg you need FTP or email the file somewhere), but doesn't explain how to fix this. My solution (I'm no expert though, so there might be a better way), is to create a package-level string variable, and at the start of the package use a script to set the value of the variable to be the required path and file name (including the time stamp). This variable can then be used to create the file and then subsequently for an FTP or email (or whatever) task.
Cheers - Graham