xs:dateTime and indexing

Given the doctype msg with three attributes, id, created-at and modifed-at.
Attribute created-at is of datatype xs:dateTime.

(Thousands of msg documents with large blocks.)

Using the following query
results in a complete document scan for my msg doctype.

The normal preselection via standard index seems not to be working for the created-at attribute.
I defined attribute modified-at of type xs:string and the same search on this attributes does use the index.

Has anyone used xs:dateTime datatype before and is there any experience with indexing of this kind of fields?
What do I have to specify in my query to avoid complete document scans when using xs:dateTime ?

Hello Frank

The following is an answer of one of our developers about your topic posted:

An index for a node with type xs:dateTime is kept in xs:dateTime order.
X-Query however does not support this data type.
The given query thus must compare according to string order, which is not implemented by
the index and consequently the index is not used
(note that it can and
will be used for supporting equality comparisons, though).

Use Tamino XQuery for a typed comparison.
Or change schema type to xs:string, when X-Query must be used.

Hope this helps
best regards