There were 42 generations, and that is from Abraham to Christ, a total of 2160 years. So when one divides 42 into 2160, it comes out to basically 51.5 years. So a generation is not 40 years as we so often said, or others have said, but 51.4.
Then when we get to Luke chapter 3, verses 23 to 38, there is 77 generations from Adam to Christ and that's 4,000 years. When one divides 77 into 4,000, he also comes out to 51.9 years. So a generation is 51 to 52 years. We begin counting this countdown to the return of Christ from the time that Jerusalem is captured by the Jews, Luke 21:24, because the 70 weeks are determined upon thy people, the Jews, Daniel 9:24. So when one adds 51.5 to 1967, when the Jews captured Jerusalem, it comes out to 2018.