I started paying for my subscription with plex about two years ago. At that time my subscriptions were due on approximately the 10th of the month. Since then, the day they are due has been moving - slowly but steadily - toward the beginning of the month. This month the subscription date finally fell over to the previous month, meaning I am buying plex at the end of this month instead of the beginning of next month.
This is expected behavior. A "month" subscription lasts 30 days, while a year lasts 365.25 days. This means that you "lose" 5.25 days per year. Below I have circled the day my subscription is due this year in red, and the days it will be due this month in subsequent years in blue. Note the 6 day gap between the 16th and the 10th, which happens because 2016 is a leap year.
We lose a month about every 6 years, and losing a full year would take place over a 70 year "grand cycle."