Cognos8-Getting coercion error while using built-in Functions.

[GeorgeBush]

Our database is dimensionally modelled. When built-in functions like _add_days are used,
coercion error occurs at the report levelthough the expression is getting validated in the expression editor.
The Expression used is {cast([Period],date)-{_add_days(cast([Period],date),-84)}}
The error displayed is" QE-DEF-0459 Invalid coercion from level to date".Here Period is the Level in a Hierarchy.

[COGNOiSe administrator]

Why curly brackets?