After noting that fees (e.g., to a dentist) will not be recognized as medical expenses until they are incurred (i.e., the related services are performed (so that, for example, fees prepaid in 2015 for services to be performed in 2016 would not qualify as medical expenses in 2015), CRA went on to confirm that:
[A]n individual may choose any 12-month period ending in the year…[but the] individual's medical expenses must have been paid…during the period so chosen. …
For example, where an individual chooses the period from January 1 to December 31, 2016…[i]f the…fees were paid on December 31, 2015, they would not be eligible for the METC computation for the 2016 taxation year…. However, these fees paid on December 31, 2015 could be included in the computation of the METC for a selected period that included December 31, 2015. If this were the case… the individual could if so desired file an application to amend his or her 2015 income tax return to include those expenses in the computation of the METC for the selected period that included December 31, 2015.