Virtual Cube. Attribute value is not a date or wrong: syntax;exception: 6931

Hello , All!

I have a problem with virtual cube on hana Model ( Analytic View) .

There are 2 native columns in table : wcreate_at  (20150409080651) and wconf_at (20150409080321) - type NVARCHAR.

I've created 2 calculated columns : create_at = time("wcreate_at") conf_at=time("wconf_at") - type TIMESTAMP

and column DURPR = secondsbetween("create_at", "conf_at") - type Integer.

Data preview editor works good :DURPR  = 210

In SAP BW virtual cube (Keyfigure type INT )) on analytic view I have en error : "attribute value is not a date or wrong: syntax;exception: 6931"

pls, help!

regards, Alexander!