To skip the first 30 seconds of a video using `ffmpeg`, you can use the `-ss` parameter. Here's a basic command to reverse a video:

```bash
ffmpeg -i input_video.mp4 -ss 30 -vcodec copy -acodec copy output_video_skipped.mp4
```

Replace `input_video.mp4` with the name of the input video file you want to reverse, and `output_video_skipped.mp4` with the desired name for the truncated output video.

---

If you would like to do both at the same time, you can use the `-ss` parameter together with the `reverse` and  `areverse` filters.  Here's an example:

```bash
ffmpeg -i input_video.mp4 -ss 30 -vf "reverse" -af "areverse" output_video_skipped_and_reversed.mp4
```
Replace `input_video.mp4` with the name of the input video file you want to reverse, and `output_video_skipped_and_reversed.mp4` with the desired name for the reversed, shortened output video. If desired, replace `30` with any other number of seconds to skip.

This command will:

-	Take the input video specified with `-i`.
-	Skip the first 30 seconds of the input video and audio with `-ss 30`.
-	Apply the `reverse` video filter with `-vf "reverse"`, which reverses the shortened video stream.
-	Apply the `areverse` audio filter with `-af "areverse"`, which reverses the shortened audio stream.
-	Save the reversed video and audio as a new file named `output_video_reversed.mp4`.

A word of caution, the ordering of the `-ss` parameter is significant.  With some versions of FFmpeg, using `-ss` as an input parameter (before `-i`) while copying, or even transcoding, employs keyframe seeking. This is much faster, but sacrifices accuracy. If you use it as an output parameter, the process is potentially much slower (the input is decoded frame by frame even for the discarded parts), but accurate.
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