# A Backward NTP Clock Step Delayed a Python Scheduler's Measurements by Nine Days

DevFeed: [A Backward NTP Clock Step Delayed a Python Scheduler's Measurements by Nine Days](<https://devfeed.tech/articles/the-ntp-clock-jump-that-made-the-scheduler-skip-a-week-of-measurements-34099.md>)

Original publisher: [Read original article](<https://philipptheserver.com/posts/clock-jump-scheduler/>)

Author: Philipp Lehmann (philipp.lehmann@gruppe.ai)

Published: 2026-09-14T07:00:00Z

Content type: tutorial

Language: en

Sources: [Philipp Lehmann](<https://devfeed.tech/sources/philipp-lehmann.md>)

Topics: [Python](<https://devfeed.tech/topics/python.md>), [Linux](<https://devfeed.tech/topics/linux.md>), [systemd](<https://devfeed.tech/topics/systemd.md>), [Single Board Computer](<https://devfeed.tech/topics/single-board-computer.md>)

Tags: [embedded](<https://devfeed.tech/tags/embedded.md>), [linux](<https://devfeed.tech/tags/linux.md>), [ntp](<https://devfeed.tech/tags/ntp.md>), [python](<https://devfeed.tech/tags/python.md>), [reliability](<https://devfeed.tech/tags/reliability.md>), [scheduler](<https://devfeed.tech/tags/scheduler.md>), [single-board-computer](<https://devfeed.tech/tags/single-board-computer.md>), [systemd](<https://devfeed.tech/tags/systemd.md>), [time](<https://devfeed.tech/tags/time.md>), [wall-clock-time](<https://devfeed.tech/tags/wall-clock-time.md>)

## AI overview

A Python scheduler on an embedded Linux device silently stopped running after NTP corrected the system clock backward by about nine days. Because it used wall-clock time to measure elapsed time, the scheduler waited roughly nine days before running again. The article explains why a monotonic clock is the appropriate fix and mentions testing with an injected clock jump.

## Source excerpt

time.time() vs time.monotonic() in a Python scheduler: a backwards NTP step delayed runs by nine days. Monotonic durations and a jump-injecting test.