IRC Section 1031 requires that taxpayers acquire all replacement property by the earlier of 180 days from the sale of the relinquished property or the Federal tax return due date for the year in which the exchange commenced. Therefore, taxpayers with exchange transactions commencing in the 4th quarter of the calendar year, may have less than 180 days to complete their exchange, unless they file an extension.
After completing the exchange, taxpayers can then file their Federal return and report their 1031 exchange transaction on IRS Form 8824.
Once a tax return is filed, it typically cannot be amended to include the exchange or for an extension of time to complete the exchange. If you file the return prior to completing the exchange, the sale of the relinquished property should be reported as a taxable transaction.
Please visit the IRS website for more information to File for an Extension
https://www.irs.gov/forms-pubs/extension-of-time-to-file-your-tax-return