Billing anchor dates

List the dates on which a subscription is actually charged within a reporting period.

A customer subscribes on start ("YYYY-MM-DD") with a free trial of trial_days days. The first charge is scheduled for start plus trial_days days. That day of the month is the customer's anchor day. Every later charge is scheduled on the anchor day of each following month; if a month is too short to have that day, the charge is scheduled on its last day instead. The schedule always goes back to the anchor day afterwards: an anchor of 31 gives Jan 31, Feb 28 (or 29 in a leap year), Mar 31, Apr 30, and so on.

A charge scheduled on a Saturday or Sunday is collected on the following Monday. This only moves that one collection date, never the schedule.

cancel_on is either an empty string (never cancelled) or a date: a charge is made only if its scheduled date is on or before cancel_on; later scheduled dates are therefore not charged either.

Input: start and cancel_on as above, trial_days (an integer, 0 or more), and from_date and to_date ("YYYY-MM-DD", from_date <= to_date).

Output: a list of dates ("YYYY-MM-DD"), in ascending order, of every charge whose collection date lies between from_date and to_date inclusive. If there are none, return [].