I wrote a script (zm_movie_cam_avconv.php) in PHP to make continuous movies from requested start/stop times in zoneminder (thanks to great comments on github to make it better)
Works with zm 1.28.1, ffmpeg and avconv
It may be useful to some (it is to me)
It can be found on github, the repository is called "zm-movie"
snapshot: /imgur*com/CumzeUz
(obviously replace * with .)