Encodings. Time Zones. Accented Characters. Daylight savings time. Do these words make you feel dread and anxiety? Among programmers, these topics are notorious for being the source of subtle, tricky bugs. Yet knowledge in this area is vital. To make this topic fresh & exciting, I created i18n-puzzles.com, inspired by the famous Advent of Code. After all, which programmer can resist a good puzzle?

I published daily puzzles during a period of 20 days in March 2025. Over 300 programmers from around the world competed. I’ll share stories from the community, which gathered to compete for the top spot on the leader board, culminating in a nail-biting finale.

Each puzzle comes with its own story, each puzzle teaches something new. Find a treasure with a UTF-8 validator, fix a banking scam by parsing right-to-left text, upgrade a time zone database in Antarctica to support science. I will pick a few of the puzzles, show solutions from the community (most of them in Python) and I will use these to illustrate common pitfalls related to Time Zones and Unicode.