NordVPN can be billed either monthly or annually, depending on the plan you choose.
Why annual plans show a monthly expiry date
If you're on an annual billing cycle, the expiry date shown inside the NordVPN client will still display a monthly date rather than your full annual renewal date. That's expected — this date automatically rolls forward by one month at a time until it reaches the end of your annual term. It doesn't mean your subscription is actually expiring monthly; it's just how the client displays progress through a longer billing cycle.