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Late dates are calculated in the backwards pass. Late dates cannot be calculated for a task path (a set of tasks connected by dependencies) that doesn’t eventually lead to a Milestone.

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Late Start

late_start The latest date a Task can start is based on when its blocked tasks are finished or started, depending on the dependency type (series or parallel).

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Late Finish

late_finish The latest date a Task can finish is based on when its dependencies are finished and its duration.

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Milestones

Unlike Tasks, Milestones have a single early_date field. This is the soonest a Milestone can occur.
In legacy CPM, Milestones have an Early Start and an Early Finish, but depending on the type, only one is used.