https://github.com/kubernetes/kubernetes/blob/87f9429087d4e31201412548517d36e83abebc8d/pkg/controller/cronjob/utils.go#L89-L149

utils.go

// from: https://github.com/kubernetes/kubernetes/blob/87f9429087d4e31201412548517d36e83abebc8d/pkg/controller/cronjob/utils.go#L89-L149

// getRecentUnmetScheduleTimes gets a slice of times (from oldest to latest) that have passed when a Job should have started but did not.
//
// If there are too many (>100) unstarted times, just give up and return an empty slice.
// If there were missed times prior to the last known start time, then those are not returned.
func getRecentUnmetScheduleTimes(sj batchv1beta1.CronJob, now time.Time) ([]time.Time, error) {
	starts := []time.Time{}
	sched, err := cron.ParseStandard(sj.Spec.Schedule)
	if err != nil {
		return starts, fmt.Errorf("Unparseable schedule: %s : %s", sj.Spec.Schedule, err)
	}

	var earliestTime time.Time
	if sj.Status.LastScheduleTime != nil {
		earliestTime = sj.Status.LastScheduleTime.Time
	} else {
		// If none found, then this is either a recently created scheduledJob,
		// or the active/completed info was somehow lost (contract for status
		// in kubernetes says it may need to be recreated), or that we have
		// started a job, but have not noticed it yet (distributed systems can
		// have arbitrary delays).  In any case, use the creation time of the
		// CronJob as last known start time.
		earliestTime = sj.ObjectMeta.CreationTimestamp.Time
	}
	if sj.Spec.StartingDeadlineSeconds != nil {
		// Controller is not going to schedule anything below this point
		schedulingDeadline := now.Add(-time.Second * time.Duration(*sj.Spec.StartingDeadlineSeconds))

		if schedulingDeadline.After(earliestTime) {
			earliestTime = schedulingDeadline
		}
	}
	if earliestTime.After(now) {
		return []time.Time{}, nil
	}

	for t := sched.Next(earliestTime); !t.After(now); t = sched.Next(t) {
		starts = append(starts, t)
		// An object might miss several starts. For example, if
		// controller gets wedged on friday at 5:01pm when everyone has
		// gone home, and someone comes in on tuesday AM and discovers
		// the problem and restarts the controller, then all the hourly
		// jobs, more than 80 of them for one hourly scheduledJob, should
		// all start running with no further intervention (if the scheduledJob
		// allows concurrency and late starts).
		//
		// However, if there is a bug somewhere, or incorrect clock
		// on controller's server or apiservers (for setting creationTimestamp)
		// then there could be so many missed start times (it could be off
		// by decades or more), that it would eat up all the CPU and memory
		// of this controller. In that case, we want to not try to list
		// all the missed start times.
		//
		// I've somewhat arbitrarily picked 100, as more than 80,
		// but less than "lots".
		if len(starts) > 100 {
			// We can't get the most recent times so just return an empty slice
			return []time.Time{}, fmt.Errorf("Too many missed start time (> 100). Set or decrease .spec.startingDeadlineSeconds or check clock skew.")
		}
	}
	return starts, nil
}