Adding days to std.datetime.Date

[via Digitalmars-d-learn]

> But still, why this method 
> http://dlang.org/phobos/std_datetime.html#.Date.add only 
> supports "month" or "years" while this one 
> http://dlang.org/phobos/std_datetime.html#.Date.roll does ?

But still, why this method 
http://dlang.org/phobos/std_datetime.html#.Date.add only supports 
"month" or "years" while this one 
http://dlang.org/phobos/std_datetime.html#.Date.roll does support 
days ?*