The date field returned by a /transactions/get request is not timezone specific and is determined by the date returned by the financial institution. Plaid does not return a definite timestamp for transactions, and only reflects the date in ISO 8601 format (YYYY-MM-DD).
If you are seeing discrepancies, keep in mind that for pending transactions, the date field reflects when the transaction occurred, while posted transactions return the date field as the date the transaction posted. For more information on different transaction states and how data is returned across them, please see our Transactions States guide.
If this did not solve your question, please file an issue in the Incorrect Transaction Data section of the Plaid Dashboard and the team responsible for our integration with the financial institution will take a closer look.