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Clock holdover error budget

Estimate worst-direction clock drift from oscillator ppm, starting error and an elapsed holdover period.

Operations

Clock holdover error budget

Estimate worst-direction clock drift from oscillator ppm, starting error and an elapsed holdover period.

Example inputs · Replace these with your measurements to build your plan.

Method, example and sources
  1. Convert ppm to accumulated milliseconds over the entered hours.
  2. Add initial absolute error in the worst drift direction.
  3. Compare the bound with the application error limit.
  4. Solve the remaining holdover duration when nonzero drift is supplied.

Worked example. Two ppm adds 172.8 ms in 24 hours. Starting 5 ms away from reference gives a 177.8 ms bound, exceeding a 100 ms budget.

Planning result · confirm model details

Error budget exceeded

The bound adds initial absolute error to worst-direction constant oscillator drift.

Added drift
172.8ms
Absolute error bound
177.8ms
Budget remaining
-77.8ms
Maximum modeled holdover
13.1944hours
  • One ppm is one microsecond per second. Enter an oscillator tolerance that includes temperature and aging. Zero ppm is an idealized scenario, not proof of an indefinitely accurate clock.
  • This does not inspect NTP/PTP state, leap seconds, time steps or source authenticity.

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How to use this result

A clock that loses its reference keeps running, but its error can grow. Parts per million describes frequency error: one ppm accumulates one microsecond each second. This worksheet adds that growth to the initial absolute error so opposite signs cannot accidentally make an optimistic plan. Enter a tolerance appropriate for the full operating environment.

What to verify next

Temperature, aging, oscillator class and synchronization behavior can change the actual bound. A zero-drift entry is only an idealized example and cannot prove indefinite holdover. Check the exact appliance specification, its current time-source state and the workload tolerance. Certificate date failures, log ordering and distributed application timing may need different error budgets even on the same network.

Worked example

Two ppm adds 172.8 ms in 24 hours. Starting 5 ms away from reference gives a 177.8 ms bound, exceeding a 100 ms budget.

Clock holdover error budget — Uniqcli