Yes, you can file up to midnight (in your timezone) Tuesday, April 18th.  (This answer applies to 2016). 

The tax deadline for your 2017 taxes is 11:59 PM April 17th, 2018, unless you take an extension, which you can do with the "Take an Extension" link in the lower left of the sidebar once you're signed in to TurboTax. 

Please note that an extension only extends when you have to file the paperwork - you still have to pay by April 17th, 2018.