Pretty sure there's a yearly off-by-one bug on at least the Happy Clock Day and Trick Or Treat medals. How they're supposed to unlock -- Happy Clock Day on Aug 15 and Trick Or Treat on Oct 31. Let's call this ProperDay
How they actually unlock (I think) -- ProperDay + (CurrentYear - 2012) Days.
So since it's 2017, I will have to try Aug 15 + (2017 - 2012) Days, or Aug 15 + 5 Days, or that is on Aug 20 and see if it unlocks then.
To be on the smart side it's probably worth trying once a day every day, from ProperDay to whatever the error adjusted day would be.