In a few months time, I had created a new calendar for our partners at Booking.com. Months? Yes, months. Creating a calendar that works correctly all across the world for 42 different languages and even more locales, is not something that you do in a week. In this talk I will talk to you about the weird things I learned. From Brazilian Summer Time and Slovakian date formatting, to surprises in open-sources libraries and dealing with wrong assumptions. And how to avoid the many (many!) mistakes I made.