“Today” feels like it should be a single, global fact, but the calendar date is actually a local property, not a universal one — it depends on where you are, not just on what moment it is. The Gregorian calendar divides the world into time zones, and each zone rolls over to the next date at its own local midnight rather than all at once.
The clearest way to see this is near the International Date Line, an imaginary boundary roughly following the 180th meridian in the Pacific. Two people can be experiencing the exact same instant in real time — the same second, measured any way you like — while one of them is already on tomorrow’s date and the other is still finishing yesterday’s. Samoa and neighboring American Samoa, for instance, sit on opposite sides of that line and can be a full calendar day apart from each other despite being close geographically. The same effect, just less dramatic, happens between any two time zones: someone in Tokyo can already be well into a new date while someone in Los Angeles is still hours away from midnight.
This is also why “what day is it” questions don’t have one universal answer the way “what time is it in UTC” does — UTC is a single global reference, but a calendar date requires picking a time zone first. The date on this page is read from your device’s own local clock and time zone settings, so it reflects your date, in your location, right now — not a fixed reference point that would be wrong for anyone reading from somewhere else.