it's a date idiom informal
（会面时间）很合适，没问题，就这么定了
used to say that a particular time is a good time to meet
"I can't make it at seven o'clock. How about 9.30?" "Sure, it's a date." “我7点钟不行，9点半怎么样？”“当然可以，没问题。”
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