it's a date used to say that a particular time is a good time to meet (会面时间)很合适，没问题，就这么定了 "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，就这么定了。” It's a date. You furnish the lunch and I'll provide the transportation. 就定那天。你负责准备食物，我提供交通工具。