How do calendars from around the world differ?

Right now, you're reading this on a particular date — maybe March 14th, or October 3rd, or some other day your calendar calls "today." But step across the ocean, and someone might be living in a completely different year. Not time-travel science fiction. Just… a different calendar. How does that work?
Here's the trick: calendars are tools humans invented to track time, and we built them using the sky. The sun gives us days and years. The moon gives us months. But the sun and moon don't line up neatly — a year is about 365.25 days, a moon cycle is about 29.5 days, and no amount of clever math makes those numbers play nicely together. So different cultures picked different solutions.
