Arcana Coelestia #1020
1020. That these things are meant becomes clear from the fact that everything put together as history from Genesis 1 down to Eber in Chapter 11 means something different from what appears in the letter, and from the consideration that the historical narratives there are purely made-up history customary among the most ancient people. When attesting the truth of some matter they would say that 'Jehovah said it'. Here however 'God' is used because the subject is the spiritual Church. And they did the same when anything true was being, or had been, put into effect.
Month
A 'month,' as in Genesis 29, signifies the end of an earlier state and the beginning of a following state, thus a new state. 'A month' signifies a full state.
(References: Arcana Coelestia 3814)