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Filtering is available on the tasks that are visible on the timeline.
The position on the timeline determines the field mapping configured (here you can read more about field mapping). The start/end dates are not related to Jira fields.
If any part of the Box/Task duration fits between start/end date filters, a task is found (partial overlap is sufficient).
Using only the start date means everything from that date onwards meets the criteria (once again, the partial overlap is sufficient).
Using only the end date means everything preceding that date meets the criteria (once again, the partial overlap is sufficient).
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