Comparing the dates in the logbooks, Tazmara concludes that the missing four pages represent one week worth of entries and should cover the period of 4..3 weeks ago, after which there is no more entries in any of the books. Checking for magic reveals that only one item in government house is magical - namely, the tampered-with logbook. It shines faint universal and, upon closer inspection, contains two arcane marks on the inside of its back cover. They are normal words and spell "PLUM" and "TREE".

Spoiler: Know (local) 10
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You know that both in Andoran and in most of the world most poisons and many other deadly things are perfectly legal. It's what you do with these things that matters.
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Hewitt know (arcana): (1d20+5)[16]

I don't think we really need absolute dates, do we? Relative ones should work just fine.