Check the size as well as the modification time.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83569 b8fc166d-592f-0410-95f2-cb63ce0dd405
Fix crazy timestamps caused by assuming the current year.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83570 b8fc166d-592f-0410-95f2-cb63ce0dd405