Thank you @arthur5005 . The leap year issue is the best guess I've heard on this topic.
I am using a twelve month period ending in the tax year that lands on the first of a month. At this point, I have been provided with the downloadable software and have proceeded with that accordingly as it absolutely allows the dates that I need.
Thanks again for your feedback.
PS I haven't been able to determine whether one can use less than a twelve month period as I suspect that most would never be in a position to want to claim less medical expenses. In a catastrophic health year, however, one might accrue more medical expenses, and therefore higher non-refundable tax credits, than one might otherwise be able to use.
