[aargh_aargh]

That trick (-ss -i -ss) should not be necessary by default (unless you use -noaccurate_seek) according to current documentation. But I haven't verified it.

https://ffmpeg.org/ffmpeg-all.html#toc-Main-options

  -ss position (input/output)

     When used as an input option (before -i), seeks in this input file to 
 position. Note that in most formats it is not possible to seek exactly, so 
 ffmpeg will seek to the closest seek point before position. When transcoding 
 and -accurate_seek is enabled (the default), this extra segment between the 
 seek point and position will be decoded and discarded. When doing stream copy 
 or when -noaccurate_seek is used, it will be preserved.
 
    When used as an output option (before an output url), decodes but discards 
 input until the timestamps reach position.