I think it should be:
restart the current track with the Skip Backwards button if the current song is played for less than x seconds
What my Panasonic does is: click skip back once -> restart current track; click skip back twice -> start previous track
Your description relates to a playtime of less than x seconds. But what happens if the song is played for more than x (e. g. 30) seconds and you press Skip Back? Is it the same as on less than x seconds - independent from the track progress?