The part about only miles earned in the same month a year ago expiring at the end of each month is fine - the rolling thing wouldn't work if they went about resetting tier miles to zero.

However, I do not believe a new rolling period will start every month you accrue 50k. Once you hit 50k, they won't review your account for 12 months. 12 months later, if you have 50k, you re-qualify for Gold.