Extend -ss and -t to support seeking to EOF time position (and backwards)?

[burek]

Maybe, it would be useful if options -ss and -t could support something like this: ffmpeg -ss EOF-00:00:10 -i input ...
(meaning the start time is 10 seconds before the end of the file).

This would be useful in automated use of ffmpeg, where users don't need to set time positions absolutely (manually), but rather relatively to the end of file.