Why Does Computer Time Drift and How We Keep It Accurate

Why Does Time Appear "Inaccurate"?

Mechanical clocks and computer oscillators both rely on crystal resonators whose frequency stability is limited by manufacturing tolerances and environmental factors such as temperature, causing gradual drift.

To compensate, modern computers automatically calibrate their clocks against network time sources.

How Does Automatic Calibration Work?

When a computer is connected to a network, it synchronizes with a "network time" service. The service is typically a time server that itself is synchronized to a highly stable reference.

However, the server’s time is not inherently perfect; it depends on a time source, often a "time server" that receives its reference from a national time‑keeping institute.

What Is the Origin of the Time Source?

Historically, time was defined by astronomical observations: the day was based on Earth’s rotation and the year on its orbit around the Sun. Early definitions divided a day into 24 hours, each hour into 60 minutes, and each minute into 60 seconds, yielding the relation 1 second = 1⁄86400 day .

Because Earth’s rotation is not perfectly uniform—its orbit is elliptical and its spin is affected by tides, mantle movements, and other factors—the length of a day varies, making the astronomical second imprecise.

From Astronomical to Atomic Time

Scientists sought a more stable periodic phenomenon and discovered that the hyperfine transition of the cesium‑133 atom occurs at a highly constant frequency, virtually unaffected by external conditions.

Measuring 9 192 631 770 cycles of this transition defines one second. In 1967 the International Committee on Weights and Measures adopted this definition, creating the International Atomic Time (TAI).

Coordinated Universal Time (UTC) and Leap Seconds

Because society still needed a time scale aligned with Earth’s rotation, UTC was defined as TAI adjusted by occasional leap seconds. When Earth’s rotation slows, a leap second is added (or rarely subtracted) so that UTC stays within 0.9 seconds of UT1 (the astronomical time).

Leap seconds keep civil time in sync with solar time.

How Computers Obtain UTC

National time‑keeping centers broadcast the official time via radio, network protocols, and telephone. Time servers receive this broadcast and redistribute it to end users.

Clients request the current time from a server; the server replies with a timestamp. Network latency, however, introduces error.

Network Time Protocol (NTP)

NTP mitigates latency by timestamping packets at four points (t1‑t4) and computing:

Network delay = (t4‑t1) ‑ (t3‑t2)

Clock offset = ((t2‑t1) + (t3‑t4)) ⁄ 2

Using these values, the client adjusts its clock. Typical accuracy is 10 ms–500 ms over the Internet and <1 ms on a LAN.

Impact on Running Programs

Most programming languages expose the system’s "wall clock" (UTC). If the clock is stepped backward during a leap second or correction, time‑dependent code can see time move backwards, causing errors.

To avoid this, many systems provide a monotonic clock that never jumps backward; it counts nanoseconds since boot.

Two Ways to Apply NTP Corrections

ntpdate : forces an immediate step to the server’s time.

ntpd : slews the clock gradually, spreading the offset over time so the clock never runs backward.

Using ntpd prevents discontinuities that could break time‑sensitive applications.

Summary

Early timekeeping relied on astronomical observations, leading to the World Time (UT). Inaccuracies caused by Earth’s irregular rotation prompted the creation of atomic clocks based on cesium, redefining the second and establishing TAI. UTC combines atomic precision with occasional leap seconds to stay aligned with solar time. National time‑keeping centers broadcast the official time, which is distributed via network time servers. Computers synchronize using NTP, which accounts for network delay and applies corrections either by stepping or slewing the clock, while developers should prefer monotonic clocks to avoid issues caused by backward jumps.