+=1 is not one second after the last tween.

It is one second after the timeline’s end — and the end is max(start + totalDuration) over every child inserted so far, not a running total. On a timeline whose first tween runs 4s and whose second ends at 0.7s, a third at "+=0" starts at 4s rather than 0.7s. Nothing in that third line mentions the first tween. It moves anyway.

The engine is free and needs no account. Paste the timeline — the defaults, the nesting, the labels and every tween with its position. There are four position spellings against two reference points: a number is the timeline’s own start, "+=n" and "-=n" are the timeline’s end, "<" and ">" are the previous tween’s start and end, and a label is a fixed time. The same offset in two of those spellings can be seconds apart. And a tween’s effective duration is not its duration — a stagger makes the last target start spread late so the tween ends at duration + spread, a repeat multiplies it, and repeat: -1 gives it no end at all, so nothing after it can be positioned from one. stagger.each grows with the target count and stagger.amount does not. Every lane ships a worked example that costs nothing. Writing the report needs credits.