Unable to understand the logic to get next schedule time for CronJob

[Fr0s1]

Hi everyone,

I'm going through Kubebuilder CronJob tutorial and is currently stuck on understanding the logic behind the function "getNextSchedule" to get the next cron schedule.

if cronJob.Spec.StartingDeadlineSeconds != nil {
	// controller is not going to schedule anything below this point
	schedulingDeadline := now.Add(-time.Second * time.Duration(*cronJob.Spec.StartingDeadlineSeconds))

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

starts := 0
for t := sched.Next(earliestTime); !t.After(now); t = sched.Next(t) {

I can only visualize the part to count the missed schedules if the earliest time is only the last scheduled time. What I don't understand is the calculation of schedulingDeadline variable (substraction but not addition) and how can earliestTime be after now and its assignment of schedulingDeadline?

I really appreciate if someone can help me shed light on this.