SQL Server 2008 R2 Date functions on NVARCHAR & VARCHAR data type

Hi, I am using SQL Server 2008 R2 and was wondering if Date functions could be used with NVARCHAR & VARCHAR data types. I created 2 Date columns one of NVARCHAR(10) an the VARCHAR(10) was able to use YEAR() & DATEPART() functions and was seeing results as expected are there any performance issue with these?

Thanks in advance...................

Ione