Auto Fill Field In A Table By Comparing A Given Date Against Current Date?

I am building a database to track contract of employees so that I can know which contracts are valid and which are expired.

My table has the following fields:

ID (Primary key)
Employee ID (Foreign key to link to the employee table)
Start_Date
End_Date
Status (Either valid or expired)
Challenge

I want when I enter the end date, the system checks the end date against the current date and fills in the status field with either valid or expired as appropriate. For instance if the contract end date is March 10,2016, the status must be filled in the word valid.