SRT subtitles format

Archived from groups: rec.video.desktop (More info?)

I am trying to build a small application to manipulate .SRT files - to
change fps, offset ... (I know there are many apps like that around,
it's just an excuse to code)
but I was unable to find a description of the SRT format:

.....

2
00:00:21,664 --> 00:00:25,200
At the end of a war
which lasted 50 years

3
00:00:25,235 --> 00:00:29,784
The Great Asian Federation has
defeated the European Federation

.....

all is obvious, apart from the 664, 200, 235, 784 indices - any hint?

Thanks!

Alessandro Magni