Create date variable for hierarchy

Hi,

if i want to create a new date variable for selecting hierarchy, this variable can only be built on 0DATE,

can somebody explain why?

I have already a variable based on an own infoobject and wanted to use this, but this is not available in the selection.

In the selection are some variables available based on 0CALDAY.

Can anyone explain me the concept behind? Why is only 0CALDAY and 0DATE available?

Or how can i still use my own variable?

BR

M.Y.