Why Does Your Computer’s Clock Drift? The Hidden Science Behind Time Sync

Hello, I’m Kaito. In this article we discuss the concept of "time" and why computers often need to call time APIs.

Why Does Time Often Run Inaccurately?

You may have noticed a household clock or a computer that, after a while, shows a noticeable drift.

Both clocks and computers rely on a crystal oscillator that vibrates at a fixed frequency when powered, but its stability depends on manufacturing quality and environmental factors such as temperature.

Because of these small errors, modern computers include an automatic calibration function.

How Is Time Calibrated?

When a computer is connected to a network, it automatically synchronizes with "network time".

Network time comes from time servers, which themselves obtain accurate time from national time‑keeping centers.

These centers broadcast time via radio waves, the internet, or telephone, and the time is then relayed to end‑users.

Where Does Time Originate?

Historically, time was defined by observing the Sun: one day equals one Earth rotation, one year equals one Earth orbit, and a day is divided into 24 hours, each hour into 60 minutes, each minute into 60 seconds.

Because the Earth’s orbit is elliptical and its rotation slows due to tidal forces and other effects, the length of a day is not constant, leading to inaccuracies in the astronomical definition of a second.

Scientists therefore sought a more stable reference and discovered that the cesium atom’s hyperfine transition provides an extremely stable frequency.

In 1967 the International Committee for Weights and Measures defined one second as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium‑133 atom.

That measurement is far more precise than any astronomical method.

The resulting "atomic clock" produces a second that is virtually identical in length, enabling the definition of International Atomic Time (TAI).

Because society still relies on the solar‑based World Time, a hybrid standard called Coordinated Universal Time (UTC) was created, combining the stability of atomic time with occasional "leap seconds" to keep UTC within 0.9 seconds of UT1 (the Earth‑rotation‑based time).

Leap seconds are added or subtracted to align atomic time with the slowly changing rotation of the Earth.

How Do Computers Synchronize Their Clocks?

Computers obtain UTC from time servers using the Network Time Protocol (NTP). NTP timestamps network packets, measures round‑trip delay, and calculates the offset between client and server:

Network delay = (t4 − t1) − (t3 − t2)

Clock offset = ((t2 − t1) + (t3 − t4)) / 2

This calculation assumes symmetric network paths.

Using this offset, the client adjusts its clock. In wide‑area networks the error is typically 10 ms–500 ms; in local networks it can be under 1 ms.

Two NTP implementations handle adjustments differently:

ntpdate : forces an immediate jump to the server time.

ntpd : slews the clock gradually, avoiding backward jumps.

Programs that rely on wall‑clock time (UTC) may experience "time‑travel" issues when the clock is stepped back; using a monotonic clock avoids this problem.

<code>t1 = time.now()
// time calibration occurs
t2 = time.now()
// what if t2 < t1?
elapsed = t2 - t1</code>

Summary

Human timekeeping began with astronomical observations, leading to World Time. Imperfections in Earth’s rotation prompted the adoption of atomic clocks, defining the second via cesium transitions and establishing TAI. UTC merges atomic precision with occasional leap seconds to stay aligned with Earth rotation. National time‑keeping centers broadcast accurate time, which is relayed to time servers and finally to computers via NTP, ensuring synchronized, high‑precision system clocks.

Understanding these mechanisms helps developers write reliable code that handles time correctly.