Is the time that you are jumping to already downloaded?
You can seek within the already downloaded portion of the media file.
To seek into the non-yet-downloaded portion of the media file, you need serverside support for streaming (RTMP {FMS, Wowza, Red5}, streamscript, streaming modules for Apache2, nginx, LightTPD).
For getting the time, see: *http://www.longtailvideo.com/jw/?search=get+time*
In particular, this thread: *http://www.longtailvideo.com/support/forum/JavaScript-Interaction/13795/Get-the-current-TIME-in-4-0-for-a-snapshot*