An online version is hosted on the github pages of this project: http://lovasoa.github.io/srtmove-js/srtmove.html (in french)
All the work is done on the client side, so the page also works offline.
The file is read with File API in javascript, and saved with the download attribute of the html anchor element.
1
00:00:01,000 --> 00:00:01,210
This subtitle has two lines
And will appear during 210 milliseconds
(see SRT file format informations on wikipedia http://en.wikipedia.ork/wiki/subrib)