Extend subtitle durations to fill gaps between consecutive subtitles.

Both values are entered in milliseconds, or in frames when the global Time code mode setting is one of the frame modes - the labels show which, and the gap column in the preview uses the same unit. Frames are counted at the current frame rate. The frame and millisecond values are remembered separately, so switching the time format never reads one as the other. The Bridge gaps step in Batch convert follows the same setting.
In millisecond mode, the … button next to each value opens a small calculator that turns a number of frames at a chosen frame rate into milliseconds - handy when a style guide gives the gaps in frames.
The preview updates live and the status text shows the number of bridged gaps.