Game1.Update C# (CSharp) Method

        protected override void Update(GameTime gameTime)
        {
            // ... other stuff ...
            asteroidSpawnTimer += (float)gameTime.ElapsedGameTime.TotalSeconds;
            if(asteroidSpawnTimer >= asteroidSpawnDelay)
            {
                asteroidSpawnTimer -= asteroidSpawnDelay; // subtract "used" time
                CreateAsteroid();
            }
        }
    }