How to update automatically a custom field base on a date

How to update automatically a custom field base on a date

Hi everyone,

I created a custom field named "Contract Status" which has status like:
  1. Subscriber
  2. Not Subscriber
  3. Retired
And also created two other fields: "Start of Contract" and "End of Contract"

Behavior wanted:
What I need is that if the field "End of Contract"  is less than the current date, then the "Contract Status" field will change to "Not Subscriber". In the same way if the field "End of Contract" is updated to a date greater than today the "Contract Status" will change to "Subscriber".

Example:
Account name: Duck Company
Contract Status: Not Subscriber
Start of Contract: 15/01/2020
End of Contract:  31/12/2020

Then an agent update the End of Contract to 31/12/2021 and the custom field "Contract Status" will change to "Subscriber".

Do not know if I am being clear with the behavior that I expect, maybe you guys could enlighten.

Best regards!