Like a real FMC, X-Plane 11's FMC will alert you when the nav data installed is out of date; nav data is marked for a specific AIRAC cycle, a 28-day period after which it is replaced.
You can fly with this older nav data - simply press "clear" on the FMC keypad to clear this message.
When you buy a subscription to Navigraph data, you get updated data for the FMS for each new AIRAC cycle.