I had no idea when I subscribed either, but my account has 690 days on it. So I back tracked some e-mails and had a debug.print in Studio to subtract 690 days from today and both came up with Jan 17th - so 688 days would actually be Jan 19thish.

(EDIT: makes sense to add 9-10 days since there are 365 days in a year instead of 360 - a closer formula would be to subtract 5 for each year subscribed and then dived by 30)