GNOME Subtitles is a good tool to edit, sync and perform all other common operations on subtitles.
Installing it is easy:
$ sudo apt install gnome-subtitles
Open up a subtitle file and you can perform the following operations:
Timings > Adjust: Give the actual times for the first and the last subtitle. All the subtitles are stretched or compressed a bit to fall within this range.
Timings > Shift: Give the time duration by which to shift all or a section of the subtitles forward or backward.
Tried with: GNOME Subtitles 1.3 and Ubuntu 14.04