[ excerpt with some modifications from author's website ]

GSubEdit, or GNOME Subtitle Editor, is a tool for editing and
converting video subtitles. It currently features read/write of
SubRip (.srt) and MicroDVD (.sub) subtitles. Framerate conversion
and frame displacement (Increase/decrease all frames by a given
offset) is also supported.

Check the WWW site below for more information.

WWW: http://gsubedit.sourceforge.net/