There is little information here, but I suspect that what you are facing is the fact that you are using a metric set based on an explicitly created data cube (a cube you created in the data cube designer), and then try to use it time dimensions within this metric set.
You need to promote instant date time hierarchies within the data cube to full Time Hierarchies in the designer, as we do not undertake any automatic operations meant to modify an entity you explicitly defined in the designer.
Things work a little bit different with auto-generated entities (i.e. if you use fields dragged directly from a database table). In such a case, when dragging a time hierarchy on top of the existing metric set, we analyze the underlying data cube, understand it is auto-generated and we promote the time hierarchy automatically for you.
This does not mean that using SQL tables is the preferred way, it just means that we never automatically modify any entities the user have explicitly defined.