Every few years, the world's timekeepers add a single second to our clocks to keep them synchronized with Earth's rotation. This "leap second" seems harmless—just one extra second in a day. But in our interconnected digital world, that single second has caused billions of dollars in damage and brought down major websites and services.
• Purpose: Keep atomic time synchronized with Earth's rotation
• Frequency: Added irregularly, roughly every 1-2 years
• Timing: Usually added at 23:59:60 UTC on June 30 or December 31
• Duration: Creates a 61-second minute instead of 60
The problem stems from a fundamental conflict between two types of time. Atomic clocks, which define our official time standard, are incredibly precise and tick at a constant rate. But Earth's rotation is gradually slowing down due to tidal forces from the Moon, making our days slightly longer over time.
To keep our clocks aligned with the Sun's position (so noon remains roughly when the Sun is highest), scientists occasionally add leap seconds. The decision is made by the International Earth Rotation and Reference Systems Service (IERS) in Paris, usually with about six months' notice.
But computer systems were never designed to handle time going backwards or minutes having 61 seconds. The results have been catastrophic:
• 2012: Reddit, LinkedIn, Foursquare, and Yelp crashed
• 2015: Cloudflare's DNS service failed globally
• 2017: Cloudflare failed again despite preparation
• Airlines: Flight booking systems have repeatedly failed
The 2012 leap second was particularly devastating. When clocks struck 23:59:60 UTC on June 30, 2012, major websites around the world began crashing. Reddit went down for hours. LinkedIn's servers locked up. Foursquare became inaccessible. The problem wasn't just with small websites—major infrastructure providers failed too.
The technical issues arise because:
Timestamp conflicts: Many systems assume time always moves forward. When a leap second creates duplicate timestamps or impossible times like 23:59:60, databases and applications crash.
Synchronization failures: Distributed systems rely on synchronized clocks. When some servers handle the leap second correctly and others don't, the entire system can become inconsistent.
Software bugs: Most programmers never test their code for leap seconds because they're so rare. Hidden bugs only surface when the leap second actually occurs.
Cascading failures: When one system fails due to a leap second, it can trigger failures in dependent systems, creating a domino effect.
The 2015 Cloudflare incident was particularly instructive. Despite extensive preparation and testing, their DNS service still failed when the leap second occurred. The problem was in a piece of code that calculated time differences—it couldn't handle the case where the same timestamp appeared twice.
Major tech companies have developed various strategies to cope:
Google's "leap smear": Instead of adding one second all at once, Google gradually slows down their clocks over 24 hours, spreading the extra second across the entire day.
Amazon's approach: AWS repeats the last second of the day instead of adding 23:59:60, so their clocks show 23:59:59 twice.
Microsoft's solution: Azure uses a similar approach to Amazon, avoiding the problematic 60th second entirely.
But these solutions create new problems. When different companies handle leap seconds differently, their systems can become desynchronized with each other, causing integration failures.
The financial impact is enormous:
• Estimated billions in lost revenue from website outages
• Trading systems have failed, affecting financial markets
• Airlines have lost bookings due to reservation system crashes
• Emergency services have experienced communication failures
The debate over leap seconds has become increasingly heated. Supporters argue they're necessary to keep our clocks aligned with astronomical time. Critics say the disruption to modern technology isn't worth the tiny astronomical benefit.
In 2022, the world's timekeeping authorities made a historic decision: leap seconds will be abolished by 2035. Instead of adding leap seconds, we'll allow atomic time and Earth time to gradually drift apart, then make a larger adjustment sometime in the future (possibly decades or centuries from now).
This decision reflects the reality of our modern world. The precision required for GPS, financial trading, and internet infrastructure outweighs the need to keep our clocks perfectly aligned with Earth's rotation.
Some interesting leap second facts:
• There have been 27 leap seconds since 1972
• All have been positive (adding time), but negative leap seconds are theoretically possible
• The last leap second was December 31, 2016
• Earth's rotation is actually speeding up slightly now, so we may not need another leap second for years
The leap second saga illustrates how our increasingly digital world creates unexpected vulnerabilities. A tiny astronomical adjustment that would have been invisible in the pre-computer age now has the power to bring down global infrastructure.
As we move toward abolishing leap seconds, we're essentially choosing technological convenience over astronomical precision. It's a pragmatic decision that reflects how dependent we've become on perfectly synchronized digital systems—and how even the smallest disruptions can have massive consequences in our interconnected world.