What causes "%d is not in cron range(0-23)" in Netflix/chaosmonkey?

Set the start hour to a valid 0-23 (24-hour clock) value in config

%d is not in cron range(0-23)

Error message

%d is not in cron range(0-23)

What it means

Sentinel-style validation error from the helper calculateDefaultCronRunHour, which computes the default cron hour as startHour minus cronBeforeStartHour. It fires when the caller (CronExpression) passes a startHour outside the allowed clock bounds (below clockStartHour or above clockEndHour, nominally 0-23); the faulty input is the out-of-range startHour integer.

Source

Thrown at config/monkey.go:475

		if err != nil {
			return "", err
		}
		return fmt.Sprintf(defaultCron, runAtHour), nil
	}
	switch cron := cron.(type) {
	default:
		return "", fmt.Errorf("%s: unexpected type %T", param.CronExpression, cron)
	case string:
		return cron, nil
	}
}

// calculates the default cron run hour based on startHour.
// The default cron starts "cronBeforeStartHour" hours
// before "startHour"
func calculateDefaultCronRunHour(startHour int) (int, error) {
	if (startHour < clockStartHour) || (startHour > clockEndHour) {
		return -1, errors.Errorf("%d is not in cron range(0-23)", startHour)
	}
	runAtHour := startHour - cronBeforeStartHour
	if runAtHour < 0 {
		// assuming a 24 hour clock system(0 - 23), -ve values means going back to previous day
		// e.g. if start hour is 0 (midnight), the "cronTime" time should be 22 hours
		// on the previous day.
		return hoursInClock + runAtHour, nil
	}
	return runAtHour, nil
}

// ScheduleCronPath returns the path to which
// main chaosmonkey crontab is located
func (m *Monkey) ScheduleCronPath() string {
	return m.v.GetString(param.ScheduleCronPath)
}

// SchedulePath returns the path to which main

View on GitHub (pinned to eaa28fb761)

Solutions

Set the start hour to a valid 0-23 (24-hour clock) value in config
Validate startHour at config load time before calling CronExpression
Check for clock conventions mixing 1-24 or 12-hour formats

Example fix

// before
startHour = 25
// after
startHour = 1

Defensive patterns

Strategy: validation

Validate before calling

func validHour(h int) bool { return h >= 0 && h <= 23 }
if !validHour(cfg.StartHour()) {
    return fmt.Errorf("startHour %d must be 0-23", cfg.StartHour())
}

Try / catch

_, err := calculateDefaultCronRunHour(startHour)
if err != nil {
    log.Printf("invalid start hour %d, defaulting to 0", startHour)
    startHour = 0
}

Prevention

Validate startHour at config load time
Use the 24-hour clock convention (0-23) in docs and schema
Add range assertions in config tests
Reject out-of-range hours before scheduling

When it happens

Trigger: CronExpression() is called with a configured start hour below clockStartHour (0) or above clockEndHour (23), e.g. startHour = 24 or a negative value.

Common situations: Config typo like startHour=2400 or 25, off-by-one from 1-12 or 1-24 style conventions, bad values from an upstream config generator.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

AI-assisted analysis of Netflix/chaosmonkey@eaa28fb761 (2026-09-03). Data as JSON: /api/errors/fa65b06be65e26e9. Report an issue: GitHub.