title: How to manually change duration field in matroska/mkv
body: <p>I have one mkv file that doesn't have valid duration.
  I want to change this duration parameter manually.
  I gone through this matroska specification defined at 
  <a href="http://www.matroska.org/technical/specs/index.html" rel="nofollow noreferrer">http://www.matroska.org/technical/specs/index.html</a></p>
  
  <p>Looking at specification for matroska this contains only identification magic numbers, but this doesn't specify length for data. </p>
  
  <p>How to parse this matroska header so that i get duration field and change this field?</p>