you would need to create a table in your database with a column for the coupon code and the end date.
Then create a recordset to lookup the entered coupon code.
Use the set session variable behavior to store the Coupon code and Expiration date from the recordset. On the Server behaviors panel, click the plus button and select eCart -> General -> Set Session Value.
Set the triger to Coupon lookuyp recordset not empty.
Set the name to couponCode
next to value, click the lightning bolt and select the couponCode column from the recordset.
Create another session, using the same trigger, but this time set the name to expDate, for the value, select the expiration date column from your database.
Now when creating the discount rule, set the trigger to:
Session variable couponCode exists
Session variable expDate exists
If today is before a certain day:
if today is before '$_SESSION['expDate']'