Tweak previous tar-mode time stamp code

* lisp/tar-mode.el (tar-header-block-summarize): Tweak previous
commit to output the time stamp in the exact same way that GNU tar
does.
1 job for master in 4 minutes and 37 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #3100
test-all

00:04:37

 
Name Stage Failure
failed
test-all Test
make[2]: *** [xdisp.o] Error 1
Makefile:402: recipe for target 'xdisp.o' failed
make[2]: Leaving directory '/builds/emacs/emacs/src'
Makefile:424: recipe for target 'src' failed
make[1]: Leaving directory '/builds/emacs/emacs'
Makefile:1117: recipe for target 'bootstrap' failed
make[1]: *** [src] Error 2
make: *** [bootstrap] Error 2
ERROR: Job failed: exit code 1