What apps to sync movie subtitles easily are available for Ubuntu?

I'm surprised no one mentioned this:

gnome-subtitles

It has impressive features, it's extremely easy to use and very well-polished. A lot of care and detail in its UI (check the char count in each line).

I use it all the time to shift subtitle timings to/from HDTV to BDRip/DVDRip releases.

Side-note: VLC player has on-the-fly subtitle timing shift. It will not change your .srt file, only the playback, but it's very convenient for movies you'll only watch once and don't care about saving the adjustments. I assign them to UP and DOWN directional keys.

With Gaupol you can edit the subtitles. Select the text you want to adjust, then go to "Tools" > "Shift Positions" and change as you like.

If you don't want to edit the subtitles:
Although it does not make a "permanent change", with VLC you can do that with the movie you are seeing. Just go to "Tools" > "Track Synchronization".

If you prefer a command-line tool, you can try subsync (requires node.js).

Note that it can also fix progressive desynchronization. This means it can fix subtitles that are late by 1 second at the beginning, but progressively get worse and are 20 seconds late at the end

disclosure: I'm the author of subsync