[phobos] datetime review

[Walter Bright]

Jonathan M Davis wrote:
>
> Really, the ideal would be to have all of it in one module but have the code 
> split up into several files. The closest way that I was aware to do that was to 
> use a single module which publicly imported the others. I'll look at splitting 
> it into an interface file and source file though.
>
>   

druntime does it for several modules, and can serve as an example.