Question 1

Are my subtitle files uploaded to a server?

Accepted Answer

No. All parsing, editing and exporting happens in your browser with JavaScript. You can disconnect from the internet and the tool keeps working — that is the cheapest way to verify it.

Question 2

Why do subtitle timings drift after trimming a video?

Accepted Answer

Because the subtitle file still refers to the original timeline. Cutting seconds or minutes off the start of a video shifts every cue by a fixed offset, so a single shift of that offset repairs the whole file.

Question 3

Why do subtitles drift progressively instead of staying in sync?

Accepted Answer

That is a frame-rate mismatch: captions timed for 23.976 fps played on a 25 fps video drift a little more every minute. Fix it with a frame-rate retime instead of a shift.