[R] lapply or data.table to find a unit's previous transaction

I have a dataset of property transactions that includes the transaction ID (TranID), property ID (UnitID), and transaction date (TranDt). I need to create a data frame (or data table) that includes the previous transaction date, if one exists. This is an easy problem in SQL, where I just run a sub-

[R] Biometric Summit - 2010

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[R] Biometric Summit - Feb. 22-25, 2010

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