A lock is not a refusal to help customers. It is the point at which helping one customer starts costing the other eleven.
Why it matters
A day that can still change at seven in the morning cannot be prepared the evening before. Technicians check their route twice because it moved. The office cannot answer what tomorrow looks like. Everyone works from a plan that is still being written, and the cost is paid in small, invisible pieces: a missed pre-call, a product not loaded, a gate code nobody looked up.
How you notice it slipping
The clearest sign is the hour. If tomorrow is still being rebuilt at half past four today, the lock exists on paper only. The second sign is that nobody can name the rule — ask three people when tomorrow is fixed and you get three answers.
What to do
Lock by work type, not by clock alone. Recurring service can settle a day or two out because it is predictable. Reactive work stays open until the end of the day before. Then write down the two or three changes that may still be made after the lock, and who may make them. A lock that allows nothing is ignored within a week; a lock that allows everything was never a lock.