Identify the kind of timeline change
A caption file belongs to a media timeline. When the picture changes, determine whether the change is one uniform shift, several removed regions, inserted material or a rearranged section. Those operations affect timing differently. A delay slider is not a general caption-editing system.
Preserve the old picture, old captions and approved new cut. Give all three clear revision identifiers. Without that relationship, a later reviewer may compare the new captions with the wrong picture and mistakenly undo a valid correction.
Create an edit-change map
Record each removed or inserted interval in the old timeline and its position in the new one. Use the editing workflow's reliable change record where available. Treat the map as evidence for proposed timing changes, then review cues around the edits.
For a hypothetical recording, removing ten seconds from the opening shifts later material by ten seconds. Removing another five seconds before a later chapter changes that later region by fifteen seconds in total. One global ten-second shift would leave the second region wrong.
Review cues that cross an edit
A cue beginning before a cut and ending after it may need splitting, shortening or rewriting. Mechanical time subtraction cannot decide whether the remaining words still correspond to the retained speech. Review the actual picture and audio at the boundary.
Likewise, reordered sections need new associations, not just a cumulative shift. Keep cue text edits distinct from timing edits in the review note. That makes it easier to explain whether a change corrected a transcription or responded to the picture revision.
Accept the reconformed delivery
Check the beginning, each changed region, a stable middle passage and the ending. Inspect speaker or sound information whose context could have changed. Then verify the final packaged caption version in the intended destination when performing acceptance.
WebVTT's timing model helps express the resulting cues; W3C WAI guides the information captions communicate. Neither source verifies the invented arithmetic case or a particular export. Retain the map and matched assets so the next picture change begins from an understandable state.
Questions answered
When is one global caption offset appropriate?
When the relationship really is a uniform shift throughout the relevant programme.
Can I subtract removed duration from every cue mechanically?
Use that as a candidate calculation, then review cues at cuts and any reordered or rewritten sections.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.