Two sync points distinguish a translation of the timeline from a change in its scale.
Equal early and late errors can be repaired by adding one signed millisecond offset to every cue boundary.
A growing error usually points to a duration, playback-speed, frame-rate or edit mismatch and needs more than a time shifter.
The full explanation
A constant subtitle offset is the same error at every point, so adding or subtracting one duration fixes the whole track. Drift changes over time. It needs a scale correction against two known sync points, often because the subtitle and media were made for different edits, speeds or frame rates. Read the complete guide for the reasoning, examples and definitions behind this answer.