How to accurately shift subtitles

There are a couple of ways to adjust timings for all subtitles in a single file with Subtitle Workshop:

1: Edit > timings > Set Delay, set a positive or negative delay time, select the "All subtitles" radio button, Apply.

2: Select all the subtitles: Ctrl+A, to add 100 milliseconds delay, press Shift+Ctrl+H, to subtract 100 milliseconds, press Shift+Ctrl+N. You can hold each of these key combinations down to rapidly increase or decrease the times.

It's much easier using Aegisub.

Just go to Timing and add/decrease the time to your subs

Then, you can play line by line to see if the edit is synced or not