The date field returned by a /transactions/sync or /transactions/get request is the same as the date returned by the financial institution on the end user's statement. The time zone used is determined by the financial institution and is not reported to Plaid. Plaid does not typically return an exact 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.