I'd say: "you must be at the party by 9 PM" (if you come any later, you won't be able to get in; you can go there at 8, 8:15, 8:30, etc, but 9 is the limit) And: "you can go to the party until 9PM"; the meaning is the same as above. If you say "you must be at the party until 9PM, it means that you are free to leave from 9:01 on.
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