Yes, you can claim any eligible medical expenses if they occurred in a 12-month period that ends in the current tax year. After you enter your medical expenses, TurboTax will prompt you to choose your 12-month claim period.
For example, the oldest medical expense that you can claim on your 2018 return is an expense from January 2, 2017, using a claim period of January 2, 2017, to January 1, 2018.
If you have a large amount of expenses in the latter part of the tax year, it might be better to save them and claim them with medical expenses you have in the early part of the following tax year. For example, if most of your medical expenses occurred after May 15, 2018, you can save them for your 2019 tax return. At that time you can use a 12-month period of May 15, 2018 to May 14, 2019, and claim all expenses that fall within that period.
You can use a different 12-month period each year; your 12-month period doesn’t have to start where your previous 12-month period ended. However, you cannot claim the same expenses more than once.