The law is not the same at morning and night.

Rules, judgments, or their enforcement may be applied differently as circumstances change. What is permitted, expected, or punished at one time may receive another treatment later, so apparent certainty about the law or other rules can be unreliable.

The saying can also criticize inconsistent authority: standards that ought to be stable may change with circumstances. It advises caution when relying on a rule as though its application will always remain uniform, rather than making a precise claim about any particular legal system.

Use it when contrasting an earlier ruling, policy, standard, or its enforcement with a later one, especially where the change makes authority seem inconsistent. It suits a reflective or critical observation, not a precise statement of legal doctrine.