Phone Hyatt and explain that you goofed up. You mistakenly believed that December 31 was part of 2018. As it ends up (for status purposes), December 31 is part of 2019. "So, here's what we need to do. Count my stay on December 31 towards 2018 status. It's a one-time exception, because I will always remember from now on that December 31 is the first day of the new year. Hyatt ends up with a happy customer, and y'all get lots of repeat business in 2019 and beyond.".
If the rep will do this, great! If not, don't make a big deal out of it. Simply HUCA (hang Up, Call Again).