tar-update-datestamp

tar-update-datestamp is a variable defined in `tar-mode.el'.
Its value is

nil

Documentation:
Non-nil means Tar mode should play fast and loose with sub-file datestamps.
If this is true, then editing and saving a tar file entry back into its
tar file will update its datestamp. If false, the datestamp is unchanged.
You may or may not want this - it is good in that you can tell when a file
in a tar archive has been changed, but it is bad for the same reason that
editing a file in the tar archive at all is bad - the changed version of
the file never exists on disk.

You can customize this variable.