I had medical expenses Jan 15, 2018 to Dec 15, 2018. The "Optimizer" chose thus the period as Dec 16, 2017 to Dec 15, 2018 which increased the amount I "owe" the CRA.

Hello,  You need to select a one year period for your medical expenses.  This can be over 2 calendar years but must be 12 months continuous.  Therefore, Dec 16 2017 to Dec 15 2018 is correct.  This will take all the expenses you have claimed and will not affect the amount you owe to the CRA.  I'd also suggest you can claim expenses from Jan 15 2018 to Jan 14 2019 (if you have more medical expenses between Dec 16 2018 and Jan 14 2019).  Here is more information on how your medical expense deductions affect your income tax: https://turbotax.community.intuit.ca/replies/2647779