In the US, where we put our months ahead of our dates 12/13/14 was the last sequential date of the 21st century. (the rest of the world, which goes from smallest to largest [i.e., day/mo./yr] had their last one about a year ago on 11/12/13)

But don’t worry, there are plenty of more exceedingly geeky math-related dates left in the century:

Fibonacci, Mersenne and Recamán are hiding in your calendar.