First tell the two kinds of misalignment apart
Misalignment comes in two kinds: a uniform offset means every cue is late (or early) by the same amount, roughly equal from start to end; progressive drift means the start is correct and the error grows toward the end, possibly reaching minutes. This article is about the second kind — for the first, see moving the whole subtitle file earlier or later.
Why it drifts more and more
Drift comes from two different clocks: the three most common causes are (1) a frame-rate mismatch — a 24fps film's subtitles on a 25fps source lose over a second per minute; (2) the video was sped up or slowed down while the subtitles kept their original pace; (3) a VFR (variable frame rate) recording whose timeline is uneven to begin with. What they share: the error grows roughly linearly with play time.
Fix it with two anchors
Because the error grows linearly, pinning a correct time at each end and re-spreading the middle proportionally puts everything back. In the timing editor, add two-anchor correction: the two "current" fields are auto-filled with this file's original times; you watch the video and enter when the first and the last cue actually appear into the two "target" fields.
How to verify the result
After running, the result panel lists each cue's original and new times side by side. Spot-check three places: the first cue, one in the middle, and the last — the three positions with smallest, medium and largest original error. If all three land, the whole file does.
When two anchors are not enough
Two-anchor correction assumes the error is linear. If the source is a VFR screen recording with a mid-file frame-rate jump, a linear pull cannot fix the middle — take an obviously off middle cue as a third point, split the file there, and run two-anchor correction on each half. Also, after fixing, rewatch the middle section at normal speed to confirm no local misalignment is left.