Adding days to std.datetime.Date

[via Digitalmars-d-learn]

On Tuesday, 2 December 2014 at 18:00:14 UTC, H. S. Teoh via 
Digitalmars-d-learn wrote:
> On Tue, Dec 02, 2014 at 05:21:27PM +0000, via 
> Digitalmars-d-learn wrote:
>> On Thursday, 7 April 2011 at 19:10:40 UTC, Piotr Szturmaj 
>> wrote:
>> >Is it possible to add a particular number of days to a Date?
>> >
>> >I have number of days since 1 Jan 2000 and I want to convert 
>> >it to
>> >Date:
>> >
>> >int days = read!int; // number of days since 1 Jan 2000
>> >Date x = Date(2000, 1, 1);
>> >x.add!"days"(days);
>> >
>> >Unfortunately add() does not support adding days. Will it be 
>> >possible
>> >in the future or is there another approach?
>> >
>> >Thanks
>> 
>> Is there any particular reason why std.datetime's "add" does 
>> not
>> support days ?
>
> Sure it does:
>
> 	import std.datetime;
> 	import std.stdio;
>
> 	void main() {
> 		auto date = Date(2000, 1, 1);
> 		date += 10.days;
> 		writeln(date);
> 	}
>
> Output:
>
> 	2000-Jan-11
>
>
> T

Indeed, thank you :)

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 ?