Anyone can use Easy Extension, even if you don't plan on finishing up with TurboTax. This free service is available to anyone with a U.S. or APO/FPO/DPO address. Get started here.
Your extension needs to be e-filed before midnight April 15, 2019 (April 17, 2019 if you live in Maine (ME) or Massachusetts (MA) due to Emancipation Day). In the unlikely event of a rejection, you have until April 20, 2019 (April 22, 2019 for ME and MA) to either resubmit or mail your extension request.
Once your e-filed extension has been accepted, you can file your IRS return anytime before midnight, October 15, 2019.
Note: Filing an extension does not give you additional time to pay owed taxes, which are due April 15, 2019 (April 17, 2019 if you live in Maine (ME) or Massachusetts (MA)) (June 15 for taxpayers out of the country). If you can't pay owed tax by the deadline, always file an extension so you can avoid the late-filing penalty, which is up to 10 times higher than the late-payment penalty.