Like @IRonMaN said, just file an extension. From IRS:
You must file Form 706 to report estate and/or GST tax within 9 months after the date of the decedent's death. If you are unable to file Form 706 by the due date, you may receive an extension of time to file. Use Form 4768, Application for Extension of Time To File a Return and/or Pay U.S. Estate (and Generation-Skipping Transfer) Taxes, to apply for an automatic 6-month extension of time to file.