The Internet does not change on June 30, 2025, but a technical important date that day affects how some systems keep time

June 30, 2025 is a leap second date — a moment when official timekeeping adds one extra second to the day. This happens because Earth's rotation is gradually slowing, and the atomic clocks that coordinate the internet run slightly faster than solar time. Without this adjustment every few years, noon would eventually drift away from when the sun is highest in the sky.

For most people using the internet, June 30, 2025 will feel completely normal. You will not notice anything different. But for the systems that run the internet itself — the servers, routers, and databases that keep everything synchronized — that extra second can cause problems if they are not prepared for it.

The leap second has caused outages before. In 2012, a leap second triggered failures at Reddit, Foursquare, and other major sites. In 2015, it caused problems with Java applications and some Linux systems. The internet industry has been debating whether to stop using leap seconds altogether, but as of now, they still happen.

Key Takeaways

  • A leap second on June 30, 2025 adds one extra second to that day so atomic time stays aligned with Earth's rotation.
  • Most internet users will not notice anything, but servers and databases that are not prepared for the extra second can crash or behave unexpectedly.
  • The leap second has caused real outages at major websites in the past, which is why tech companies now test their systems in advance.
  • The internet industry is considering eliminating leap seconds in the future, but they are still in use as of 2025.

Why the internet needs to know about leap seconds

The internet runs on Coordinated Universal Time (UTC), a global standard that keeps all servers and devices synchronized. UTC is based on atomic clocks, which are extremely precise but do not match Earth's actual rotation exactly. Atomic time runs slightly ahead because Earth spins more slowly than it did centuries ago.

Every few years, timekeepers add a leap second to UTC to keep it aligned with solar time — the time based on the sun's position. This keeps noon close to when the sun is actually highest in the sky. Without leap seconds, over thousands of years, noon would drift to a different part of the day.

The problem is that most computer systems were not designed to handle a time jump. When a leap second happens, the clock goes from 23:59:59 to 23:59:60 to 00:00:00 the next day. Many systems do not have a 23:59:60 in their code, so they either crash, freeze, or behave unpredictably when that second arrives.

What happened during past leap seconds

The 2012 leap second caused Reddit, Foursquare, StumbleUpon, and other sites to go down for hours. The problem was in a Linux kernel issue that caused systems to get stuck in an infinite loop when the leap second occurred. Many companies had not tested for it because leap seconds are rare — they happen roughly every 18 months, but the exact date is announced only six months in advance.

In 2015, a leap second caused problems with Java applications and some financial trading systems. The issue was similar: code that assumed time always moved forward in a predictable way broke when time jumped backward by one second (from the system's perspective).

These outages taught the tech industry that leap seconds are a real risk. Now, major companies like Google, Amazon, and Meta test their systems before each leap second date. Many also use a technique called leap smearing, where they spread the extra second across several hours instead of adding it all at once, so no system experiences a sudden time jump.

How major tech companies are preparing for June 30, 2025

Google has been using leap smearing since 2012. Instead of adding one full second at midnight, Google's servers gradually slow down over an 18-hour period around the leap second, adding tiny fractions of a second at a time. This way, no single system experiences a sudden jump, and applications do not break.

Amazon Web Services, Microsoft Azure, and other cloud providers have similar strategies. They test their systems weeks in advance, patch any code that might fail, and monitor their infrastructure closely on the day itself. Most of these companies publish advisories before leap seconds so their customers know what to expect.

Smaller companies and older systems are more at risk. If you run a website or process on older hardware or software that has not been updated in years, the leap second could cause problems. The safest approach is to test your systems now if you manage any internet-facing infrastructure, and to contact your hosting provider to ask what they are doing to prepare.

What you should do if you manage a website or server

If you run a website, process, or any system connected to the internet, check with your hosting provider or cloud service about their leap second plan. Most major providers have already published their strategy. If you manage your own servers, update your operating system and software to the latest versions, as patches for leap second issues have been available for years.

Test your systems if possible. Run a simulation of what happens when time jumps forward by one second, or contact your software vendor to ask if they have tested for leap second compatibility. For most modern systems running current software, this will not be a problem — but it only takes one unpatched server to cause an outage.

If you are a regular internet user with no technical responsibility, you do not need to do anything. Your device will handle the leap second automatically, and the websites you use will either handle it smoothly or experience a brief outage that their engineers will fix quickly.

The debate over whether leap seconds should exist at all

The leap second has become controversial in the tech industry. Some argue that the cost of maintaining leap seconds — the testing, the outages, the complexity — is not worth the benefit of keeping UTC aligned with solar time. Others point out that most people do not care whether noon is exactly when the sun is highest; GPS, for example, already ignores leap seconds.

In 2022, the International Telecommunication Union voted to eliminate leap seconds by 2035. This would mean UTC would slowly drift away from solar time, but it would also mean the internet would never have to deal with leap second outages again. However, this decision has not been finalized, and leap seconds are still in use as of June 2025.

Until that change takes effect, leap seconds will continue to happen. The next one after June 30, 2025 is not yet scheduled, but timekeepers will announce it at least six months in advance if one is needed.

Frequently Asked Questions

Will my phone or computer be affected by the leap second on June 30, 2025?

Almost certainly not. Modern phones and computers update their time automatically from internet time servers, and they handle leap seconds without any visible change. You will not notice anything different on that day.

Can the leap second cause my internet to go down?

It is possible but unlikely. Major websites and cloud providers have been preparing for leap seconds since 2012 and have systems in place to handle them. Small or outdated websites might experience brief issues, but widespread internet outages from a leap second are rare now.

Why does the leap second happen at midnight UTC instead of some other time?

Midnight UTC (which is different times in different time zones around the world) is chosen because it is the least disruptive moment. It happens when the fewest people are actively using the internet and when financial markets are closed. This minimizes the impact if something does go wrong.

If leap seconds are eliminated in 2035, what happens to the time difference between UTC and solar time?

UTC would slowly drift away from solar time. Over centuries, noon would gradually shift to a different time of day. However, this would not affect how the internet works or how people tell time in their daily lives — it would only matter to astronomers and people who care about the precise relationship between atomic time and Earth's rotation.

Do I need to update my software before June 30, 2025?

If you use a major operating system like Windows, macOS, or a current Linux distribution, you should already have leap second patches from regular updates. If you manage a server or run specialized software, check with your vendor or hosting provider to confirm they have tested for leap second compatibility.