Correct, you have until the 18th to file (electronically or postmarked US mail). There is no penalty if you're getting a tax refund, provided you file within 3 years of the April 18, 2017 deadline (or October 16, 2017 deadline if you filed an extension). After 3 years, unclaimed tax refunds are forfeited and become the property of the U.S. Treasury. If you owe taxes, pay what you estimate your tax burden will be and file an extension, you will avoid the heavy 5% penalty.
