Just got this in my inbox this morning.
Grab your free discount code: LOVESUN1
Only today from 9h00 until 15h00: a limited amount of discount codes are available today at OUI.sncf for your travels between 6/05 and 30/06.
-20€ (1) discount
So the purchase time limit is in European time. that means you have 2 hours to act.