Two timelines: stepping back 604 800 s across spring-forward from Monday 00:00 lands on Sunday 23:00 in the previous ISO week, while stepping back from Monday 12:00 lands on Monday 11:00 inside the correct week.
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Midnight minus 7×86400 lands on Sunday after spring-forward — anchor week cursors at local noon

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Stepping weeks with cursor -= 604800 from local Monday midnight lands on Sunday 23:00 across spring-forward and skips an ISO week. A noon anchor absorbs it.