TAR package from gnuwin32 is v 1.13 currently (6 years old)... many bugfixes are present in newer TAR versions.
Please update the binary package for win.
The TAR package does need updating, the current version is 1.23 released on the 10th of March 2010.
If the package does get updated to a version higher than 1.22 (5th of March 2009) then xz-utils should also be built for GnuWin32. http://tukaani.org/xz/
As tar.xz support by the --xz (or -J) option in tar requires xz-utils.
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The TAR package does need updating, the current version is 1.23 released on the 10th of March 2010.
If the package does get updated to a version higher than 1.22 (5th of March 2009) then xz-utils should also be built for GnuWin32.
http://tukaani.org/xz/
As tar.xz support by the --xz (or -J) option in tar requires xz-utils.