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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