Ex3 - how to test it

import java.time.DayOfWeek;
import java.time.Instant;
import java.time.LocalDateTime;
import java.time.ZoneId;

public class SubscriptionService {

    private final BillingGateway billingGateway;

    public SubscriptionService(BillingGateway billingGateway) {
        this.billingGateway = billingGateway;
    }

    public void processSubscription(String userId) {
        LocalDateTime now = LocalDateTime.ofInstant(
                Instant.now(),
                ZoneId.systemDefault()
        );

        boolean isWeekend = now.getDayOfWeek() == DayOfWeek.SATURDAY
                || now.getDayOfWeek() == DayOfWeek.SUNDAY;

        if (isWeekend) {
            billingGateway.applyWeekendDiscount(userId);
        } else if (now.getHour() >= 22 || now.getHour() < 6) {
            billingGateway.applyNightDiscount(userId);
        } else {
            billingGateway.applyStandardRate(userId);
        }
    }
}