2.9 Shift subtitles
This feature moves all the selected subtitles initial time a specified number of milliseconds forward/backwards, without modifying their duration. It is very useful to easily modify timings to synchronize with a movie. To shift selected subtitles forward, click the "Edit/Timings/Shift +X milliseconds" menu or the [Shift]+[Ctrl]+ keys. To shift selected subtitles backwards, click the "Edit/Timings/Shift -X milliseconds" menu or the [Shift]+[Ctrl]+[N] keys.
You set X in the settings.
I think with the split & join subtitle files feature this should let you do that ?