Q.01
Why do subtitles drift out of sync?
Because the subtitle file and your video copy come from different sources. The subtitles were timed against one version of the film; you are playing another. The most common culprit is a framerate mismatch: a film released at 23.976 frames per second and a PAL version at 25 fps run at slightly different speeds, so every second of video stretches or shrinks the gap a little more. Different cuts, removed logos and trimmed intros do the rest. The result is the classic symptom: subtitles that start fine and are seconds off by the end.
Q.02
Why is a constant shift not enough?
A constant shift moves every cue by the same amount, which only helps when the whole file is uniformly early or late. Drift is a different animal: the error grows over time, so a shift that fixes minute 5 ruins minute 85. The correct repair is a linear one, a scale plus an offset. DriftFix computes it from two anchor points: you tell it when one early line and one late line should actually appear, and it retimes every cue in between proportionally. Both anchors land exactly where you put them, and everything else falls into place.
Q.03
What is a framerate mismatch, in plain terms?
Film is usually shot at 24 or 23.976 frames per second. European TV runs at 25. Instead of converting properly, PAL releases traditionally just play the film 4 percent faster. A 100 minute film becomes roughly 96 minutes, and subtitles timed for the original slide further behind every minute. If you know the two rates involved, the fix is a single multiplication, which is what the one-click presets do: 23.976 to 25, 25 to 23.976, 24 to 25 and friends. If you do not know the rates, the two-anchor method measures the drift for you.
Q.04
Is my subtitle file uploaded anywhere?
No. The file is read, parsed, retimed and rebuilt by JavaScript running in your browser. There is no upload, no server-side processing, no account, and no copy of your file anywhere but your own machine. If you want to verify that, load this page, disconnect from the internet, and fix a file. It works exactly the same, because there is no server in the loop.
Q.05
Which formats does it support?
SRT (SubRip), VTT (WebVTT) and ASS/SSA. The format is detected from the file contents, not just the extension. Your text, formatting tags, cue settings and styling are preserved byte for byte; only the timestamps change. SRT output is renumbered and sorted, VTT NOTE and STYLE blocks pass through untouched, and ASS files keep their styles and headers exactly as they were. The fixed file downloads in the same format it came in, with .fixed added to the name.
Q.06
The subtitles are still off after fixing. What now?
Two things to check. First, pick anchors as far apart as possible: one of the first spoken lines and one of the last. The wider the span, the more precise the correction. Second, make sure your video copy is a single consistent cut. If scenes were added or removed mid-film, no linear correction can fix both halves at once; fix the file with anchors placed inside the half you care about, or fix each half separately.