If you are referring to T4 slip Box 85 for employee paid premiums for a health service plan: medical expenses can be grouped into any consecutive twelve month period. The expenses in that 12-month period can then be claimed on the tax return for the tax year in which the 12-month period ends.
The same twelve month period starting date and ending date is not required year after year, and any two periods can overlap. However any particular expense is claimable only once. TurboTax will prompt you to choose the twelve month claim period after the medical expenses have been entered.
So a particular medical expense incurred in 2019 can be deferred to a return for the 2020 tax year if the chosen 12-month period ends in 2020 and the particular expense falls within that 12-month period. For example if the chosen 12-month period is July 1, 2019 to June 30, 2020, a medical expense incurred on July 1 2019 can be claimed on the return for the 2020 tax year (but not on a later return.)