For medical expenses, you can claim expenses incurred in any continuous 12 months ending in the tax year. The flexibility is, you can choose any 12-month window that you have the most expenses to claim. For example, if you haven't claimed any medical expenses on your 2015 return, on your 2016 return, you can claim expenses incurred in the period from Feb 1, 2016, to Jan 31, 2017.
According to tax guide, all medical expenses you claimed for yourself, your spouse and your dependents must be within the same claiming period.