For medical expenses, you can claim expenses incurred in any continuous 12 months ending in current 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 2018 return, then on your 2019 return, you can claim expenses incurred in the period from Feb 1, 2018, to Jan 31, 2019.
According to tax guide, all medical expenses you claimed for yourself, your spouse and your dependents must be within the same claiming period.