Correct XTAF time parsings

This corrects an issue where XTAF times were being reported one month
too large.  Timestamp numerical values now agree with JPEG-recorded
times on an experimental drive being prepared for release.

This fixes:
<https://github.com/ajnelson/sleuthkit/issues/11>

Signed-off-by: Alex Nelson <ajnelson@cs.ucsc.edu>