Help:About the function _add_years() in FrameWork Manager

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I wanna add a calculation fields in FrameWork manager.
For instance:[current_date]=2009-12-21

so run the function :
[_add_years(current_date,-1)]
return value:[Nov 21 2008]

But,we run another function:
[_add_years(cast('2009-12-21',date),-1)]
return value:[Nov 24 2008]

why return different values?