How Clocks Stay Accurate: Atomic Time and the Internet
Every clock you trust traces back to atomic clocks in the end. Time servers copy their time and pass it around the internet, your phone or computer checks in with those servers to correct itself, and a website clock then puts your time zone on top. Most online clocks simply show your device's time, so they're only as good as that device. ClockDen goes a step further and checks itself against internet time when the page opens.
What makes an atomic clock so steady
It counts the natural vibrations of atoms, usually caesium. By international definition, one second is exactly 9,192,631,770 of those vibrations, and since every caesium atom vibrates at the same rate, atomic clocks keep time far more steadily than any watch could. Hundreds of them around the world are averaged together to make UTC; see what UTC is for the rest of that story.
How your phone gets the time
It doesn't really know the time by itself. It asks a time server using a standard internet method called NTP and adjusts to the answer. Phones can also pick the time up from the mobile network and from GPS satellites, which carry atomic clocks of their own, and that's why a phone that's online is usually right to within a fraction of a second.
Why a computer clock goes wrong
The most common cause is a wrong time zone or region setting. The time is actually correct, just for the wrong place, and that's nearly always the reason a clock looks exactly an hour off. Next comes automatic time being switched off; a cheap computer clock left to itself can gain or lose a few seconds a day, and that adds up. The third is a device that hasn't been updated. Countries change their daylight saving rules, and a device without the latest update will switch on the wrong date.
If your clock is only a few seconds out, it's almost always the second one, so turn automatic time on and sync again.
How accurate an online clock really is
Many website clocks just show your device's time, converted to the place you pick, so if your device is off, so is the page. ClockDen asks an internet time source for the time, allows for how long the answer took to arrive, and corrects the difference, so a device that's a few seconds out doesn't matter here. For anything that has to be exact, like a ticket sale opening or a deadline, it's still worth turning on automatic time on your device too. You can open any city from the countries list or check UTC now.
The time zone, by the way, doesn't affect accuracy at all. It changes the label on the moment and nothing else.
When countries change the rules at short notice
Time zone rules are set by governments, and governments change them, sometimes with only a few weeks' warning. Software relies on a shared database of those rules that has to be updated each time, which is the best argument for keeping your devices up to date. For legal or travel decisions, confirm with an official source.