ffmpeg - video with timer
This is possible but will require scripting. ffprobe will be used to determine the duration of the input, then the countdown and background box will be created by the drawtext filter.


A 30 second input will start from 30 and count down to 0.

Example:

input=input.mp4
duration=$(ffprobe -loglevel error -show_entries format=duration -of default=nw=1:nk=1 "$input")
ffmpeg -i "$input" -vf "drawtext=fontfile=OpenSans-Regular.ttf:text='%{eif\:$duration-t\:d}':fontcolor=white:fontsize=24:x=w-tw-20:y=th:box=1:boxcolor=black@0.5:boxborderw=10,format=yuv420p" -c:v libx264 -c:a copy -movflags +faststart output.mp4



If you prefer rounded corners you may be able to make a PNG file with alpha, and then place it in your desired location using the overlay filter. Doesn't seem worth it to me though.
If you find the duration from ffprobe to be inaccurate then you will have to completely decode the file to get the actual duration and then parse the console output. One method would be: ffmpeg -i input -f null -.
This may be possible to do without ffprobe but I don't have an alternative method at the moment.
You can combine this filterchain with the one from ffmpeg drawtext filter - create transparent background with text to get exactly what you want. An oversimplified command could then look like:

-vf "format,drawbox,drawtext,drawtext,format"