we agreed on a date for the next meeting,（那个on）

we agreed on a date for the next meeting,（那个on）
we agreed on a date for the next meeting,（那个on）

we agreed on a date for the next meeting,（那个on）
we agreed on the date for the next meeting
但最好把a 换成the,因为date 有修饰词,下次会议或见面的日期,所以用the特指

对的，字典里的例句也是这样的

we agreed the date for the next meeting.