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Clock ​

A computer uses internal clocks to represent the system time.

Hardware clock ​

  • Also called Real-Time Clock (RTC)
  • Small clock circuit on the motherboard
  • Typically use quartz crystal oscillator
  • Allow to keep track of time when the computer is off
  • Powered by a backup battery
  • Pass the time to the OS so it has a starting reference point

System clock ​

  • Also called software clock or kernel clock
  • Handled by the OS when it's running
  • Often adjusted and synchronized (see below)

Synchronization ​

Problem ​

  • Hardware and software clocks are subject to drifts. It loses accuracy over time because of physical and electrical factors (temperature, voltage changes, etc)
  • It can lead to inconsistent timestamps, making log correlation, debugging, backups or distributed transactions harder
  • Can cause security and authentication issue (time-based tokens invalidation)

Solution ​

  • Time servers are reliable sources used to synchronize the clock, using references like GPS or atomic clocks
  • Network Time Protocol (NTP) is used over an IP network to synchronize time. A remote time server is interrogated about the current time
  • The network delays and offset are estimated to adjust the local clock accordingly