If a COBRA payment was made on 12/26/17 for our Jan 2018 coverage do I include that as part of my 2017 medical expenses or would payment be itemized on 2018 tax return?

My husband became unemployed in September 2017, we have been paying COBRA premiums to maintain our health policy.  All payments are due 1st of the month so checks are cashed 4-5 days prior.  The payment made on 12/26/2017 was for January 2018 coverage so I'm not sure if I include that payment as 2017 expense since it was for our 2018 coverage.