Unless of course your backend is in SQL server, forget Transact SQL... You'll have to use Jet functions to accomplish the same thing. IIF function and Isnull is waht you need. Isnull tells you whether or not a value is null in JET. The equivalent funtion to the Transact Isnull in jet is NZ (null to zero). And Jet uses the hash (#) as the delimiter for dates...
Select Employee.ID, IIF(isnull(Employee.hire_Date) = yes, #12/31/2020#, Employee.hire_date End) AS Hire_Date
from employee;