I want to create a table which has hiredate and Interval of hiredate and sysdate. How to create a column that automaticlaly stores the calculation.
I don't believe you can.
The expression you use to create a virtual column must be deterministic, i.e. always return the same value when called with the same arguments. Involving the current date is fundamentally incompatible with that requirement.
You could use a plain old view though.