Understanding -itsoffset option
The itsoffset option applies to all streams embedded within the input file. Therefore, adjusting timestamps only for a single stream requires to specify twice the same input file.
ffmpeg -i INPUT -itsoffset 5 -i INPUT -map 0:v -map 1:a OUTPUT
adjusts timestamps of the input audio stream(s) only.
Understanding a positive offset
ffmpeg -itsoffset 5 -i INPUT OUTPUT
delays all the input streams by 5 seconds.
If the input file was 60 seconds long, the output file will be 65 seconds long. The first 5 seconds will be a still image (first frame).
Understanding a negative offset
ffmpeg -itsoffset -5 -i INPUT OUTPUT
advances all the input streams by 5 seconds.
This discards the first five seconds of input. However, if the input file was 60 seconds long, the output file will also be 60 seconds long. The last 5 seconds will be a still image (last frame).