I don't understand why the framerate of the video should affect timestamps in SubRip (*.srt) subtitles. If I am correct, when you convert a video from one framerate to another, the length of the video remains the same—the only thing which changes is the total number of frames. But tools which change the subtitle framerate seem to follow an equation like
timestamp_1 x framerate_1 = timestamp_2 x framerate_2.
Which seems to imply that the frame numbers are equal. For instance, in the SRTLab documentation, in order to convert from PAL (25.000 fps) to NTSC (23.976 fps), it says that each timestamp should be scaled by a factor of 25.000/23.976. I don't understand this. This would make the subtitles appear after the same number of frames in both videos, which is not what we're after. Obviously, I'm not getting something right.