Re: init DateTime is very annoying because we always need to initialize the DateTime object

[slangmgh]

@GULPF

I need to define the variable first, then initialize the object later according 
to different condition. And when other object reference this object with 
`DateTime`, they need to be initialized when variable define if we don't want 
to see the warning.

In nim, variable can be defiend as: 
    
    
    var obj: T
    

But with the `DateTime`, it gives warning.

Maybe implementation of `DateTime` should make the month start from `0`, then 
everything seems fine.