IRS 1040NR instruction:

If you did not receive wages as an employee subject to U.S. income tax withholding, file Form 1040NR by the 15th day of the 6th month after your tax year ends. A return for the 2017 calendar year is due by June 15, 2018. If you cannot file your return by the due date, file Form 4868 to get an automatic 6-month extension of time to file.

Comment: Misleading to sound like extension works to 12-15, whereas IRS 4868 instruction & Pub. 54 indicate only until 10-15 when letter request not made.

IRS 4868 instruction:

Generally, we can’t extend the due date of your return for more than 6 months (October 15, 2018, for most calendar year taxpayers). However, there may be an exception if you’re living out of the country. See Pub. 54 for more information.

Pub. 54

In addition to the 6-month extension, taxpayers who are out of the country can request a discretionary 2-month additional extension of time to file their returns (to December 15 for calendar year taxpayers). To request this extension, you must send the Internal Revenue Service a letter explaining the reasons why you need the additional 2 months. Send the letter by the extended due date (October 15 for calendar year taxpayers) to the following address: Department of the Treasury Internal Revenue Service Center Austin, TX 73301-0045 You will not receive any notification from the Internal Revenue Service unless your request is denied.