Why is the time on my work computer slower than that on my phone?

It's almost time to get off work. I packed up my things and prepared to rush out, but I found that the time on my computer at work is 2 minutes slower than that on my phone? ! I wonder if you have ever had this experience. How is time measured? Why is the time on different devices different? Today, let's talk about time.

01

How is time measured?

Many people may have such questions: How is time measured? How is one second defined? In fact, human cognition of time has gone through multiple stages, and this way of cognition is influenced by many factors such as culture, technology, and social development. In the early days, the ancients mainly kept time by observing natural phenomena , such as the alternation of day and night, the change of moon phases, and the change of seasons. These observations provide a basic concept of time. The most classic one is the twelve hours summarized based on the activity patterns of the sun, daily production activities, and living habits. In addition, the ancients also used water clocks, hourglasses, etc. to assist in measuring time, but these measurement methods lack accuracy.

Image source: Zhihu

With the development of science and technology, people began to use mechanical clocks to measure time. The earliest mechanical clock was a mechanical oscillating clock, which measured time by the back and forth oscillation of the pendulum. The more stable the swing period, the higher the measurement accuracy.

Image source: pixabay

With the development of atomic physics, people began to use atomic clocks as the standard of time. In 1967, the International System of Units linked the definition of the second to the oscillation period of the cesium atom , and the second was defined as "9,192,631,770 times the period of the ground state hyperfine structure transition of the cesium atom". Such a definition has a high degree of stability and repeatability because the oscillation frequency of the cesium atom is very stable and easy to measure.

02 Why are electronic clocks often a few minutes slow?

Before mobile phones became ubiquitous, people probably woke up with electronic clocks every morning. Maybe some of you still use them now. Electronic clocks are often a few minutes slow, and the time needs to be readjusted every once in a while. Why is this?

Image source: Photo Network

In fact, the core device of the electronic clock is the quartz crystal oscillator , or crystal oscillator for short. When the electronic clock is powered on, the crystal oscillator will generate a very stable oscillation frequency (generally speaking, the oscillation frequency of the crystal oscillator is very high). The circuit inside the electronic clock will divide the frequency of this high-frequency signal to obtain a low-frequency signal that is more suitable for timing. The built-in counter of the electronic clock then counts this low-frequency signal to measure the elapsed time, and finally displays the time on the electronic display.

Piezoelectric strain gauges generate voltage when they change shape (exaggerated diagram)

Image source: wikipedia

The main sources of error of electronic clocks are as follows:

-Temperature sensitivity of quartz crystals

The oscillation frequency of quartz crystal is affected by temperature, and temperature changes may cause errors in electronic clocks. Some advanced electronic clocks are equipped with temperature compensation circuits to reduce this error.

-Battery level changes

Portable electronic clocks are usually powered by batteries, and reduced battery power may cause changes in the oscillation frequency, which in turn causes errors.

- Manufacturing precision

Factors such as the precision of electronic components and the quality of oscillators will also affect the accuracy of electronic clocks.

-Calibration issues

Long-term use may cause aging of the internal components of the electronic clock, and regular calibration is required to maintain accuracy.

- Environmental factors

Electronic clocks may be affected by environmental factors such as electromagnetic interference and vibration, causing a certain degree of error.

03

Why are the times different for devices that are connected to the same network?

In life, we can see the shadow of time everywhere, such as the microwave oven at home and the elevator in the office building. These devices do not use crystal oscillators to time like electronic clocks, but use other methods. Traditional microwave ovens can realize the timing function through the stable frequency of the power grid . The frequency of China's power grid is 50Hz. As long as the microwave oven finds that the alternating current has changed for 50 cycles, it can automatically record it as 1 second . In fact, the electronic timing of many household appliances depends on the uniform frequency of the power grid. The frequency changes with the load of the power grid and is calibrated once every 24 hours to ensure that these clocks remain accurate for a long time. However, if the frequency continues to drop for a period of time, it will cause the electronic timing of central heating timers, microwave ovens and other equipment to slow down. For example, in 2018, electronic clocks in many European countries "ran slowly" by 6 minutes. This phenomenon lasted for more than a month. Finally, it was found that the root cause was insufficient power supply .

Image source: Photo Network

If it is a smart microwave oven or smart elevator, they have built-in network cards and can get the time from the Internet like a computer. But why do devices that are also connected to the Internet also have time errors? There are many reasons for time inconsistency. Here are some possible factors:

- Network latency

It takes time for data to travel over the network, and network latency can vary from device to device. Even a tiny network delay can cause a difference in time between devices.

- Differences in time synchronization protocols

Different devices may use different time synchronization protocols. Even common time synchronization protocols (such as NTP) may be configured or implemented differently on different devices, resulting in inconsistent time.

- Hardware clock drift

The hardware clock on a device may drift slightly due to manufacturing differences, temperature changes, etc., which can cause the time to be slightly offset.

-Regular time synchronization

Some devices synchronize their time regularly, while others do not have this feature enabled, which can also lead to differences in time between different devices. So when you can rush out after get off work, you still have to look at the time on the clock in machine~ Author: Yin Guangshui, Master of Communication Systems, EPFL

Planning & Editing: Ding Dong

Acknowledgements: Xiaoping Liu, a lecturer at the School of Data Science and Engineering, East China Normal University, provided scientific guidance for this article