If you are allowed to write your own stored procedures for reporting, I would do it there. You could pass a value for the units to the DATEDIFF function, so that if you chose "years" it would use 'yy' and if you chose quarters, it would use 'qq'.

For more on DATEDIFF, check out this page: http://msdn.microsoft.com/en-us/library/ms189794.aspx