To me they are pretty different. "A will happen once X" to me means X is a necessary and sufficient condition for A happening. "B can't happen before X" impli

To me they are pretty different. "A will happen once X" to me means X is a necessary and sufficient condition for A happening. "B can't happen before X" implies that X is a necessary condition, but it does not imply sufficiency to me. If someone says: "I will give you a million dollars once you do a dance", I understand that as a commitment to provide me with a million dollars after I do a dance. If someone says: "I can't give you a million dollars before you do a dance", I understand that dancing is a requirement for getting the million dollars, but I understand there may be other requirements as well (or they may not be willing to provide the money at all).