As to Dogen's conception of Being-Time. I can give that to you in a nutshell. I wanted to point you to a scholarly treatment to read too, but I cannot access it on the internet. So, you will just have to do with me. Here is my quick version. It helps to realize that Dogen was always proposing Reality from several perspectives at once, some seemingly contradictory (X exists, X does not exist), but just different vantage points, each true in its way:
30. To you who say that the ShŰbŰgenzŰ is difficult to understand
Master DŰgen doesnít expect anything from us thatís not humanly possible. Itís simply a matter of becoming natural, without empty thoughts or peculiarities. Buddhism in general doesnít demand anything special from us, only that we become natural. Some verses in the sutras might seem special to us, for example, ďThe white hair between his eyebrows illuminates the 3,000 worlds.Ē But thatís only a literary symbol for the samadhi that is the king of all samadhis.