As it is, I have no idea how this should work.

The way I expected it was something like this:

There should be 2 spinedits (i.e. one more) which should be labelled 'Current time coordinate' and 'Time shift'

So that, I can specify at (around) what time the time shift sould be applied.

There's more:

The radiobutton captions should be 'Until' and 'After'.

And, there's yet more: The time shift itself need to have a sign (i.e., are we increasing the times or decreasing them?).

SE has moved to GitHub - please re-add this issue at GitHub if it's still important :) https://github.com/SubtitleEdit/subtitleedit/issues