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Clock skew and drift

Category: Distributed systems · Status: stub

One line: Skew is how far apart two machines' clocks are at one moment, drift how fast they move apart — the reason timestamps from different machines cannot reliably order events.

Also called: clock drift, clock synchronization, NTP, PTP, TrueTime.

How it connects

flowchart LR
  n_clock_skew["Clock skew and drift"]
  n_logical_clocks["Logical clocks"]
  n_clock_skew ---|or| n_logical_clocks
  classDef center stroke-width:3px
  class n_clock_skew center
  classDef outside stroke-dasharray: 4 3
  class n_logical_clocks outside

In each language
Rust Instant ↗ is monotonic; SystemTime ↗ is not, and is the one for communicating with other processes
Go time.Now returns both a wall-clock and a monotonic clock reading ↗, and the monotonic reading has no meaning outside the current process
C++ std::chrono::steady_clock ↗ is monotonic: its time points cannot decrease as physical time moves forward
Java System.nanoTime ↗ measures elapsed time only and is not related to wall-clock time
Python time.monotonic ↗ cannot go backwards and is not affected by system clock updates
Erlang and Elixir the runtime keeps Erlang monotonic time apart from system time, with time warp modes ↗ for when system time changes
The operating system CLOCK_REALTIME ↗ jumps when the system time is changed; CLOCK_MONOTONIC cannot be set

Where to read more