Meeting finder

Find every time slot in a working window where a meeting of a given length fits, given everyone's calendars.

Each calendar is a list of busy intervals written "HH:MM-HH:MM" (24-hour clock, same day, start not later than end). Intervals from different calendars or the same calendar may overlap, nest or touch. All calendars are combined: a minute is busy if it lies in any interval.

Rules:

• An interval whose start equals its end is empty and is ignored completely (it gets no buffer either).
• Every other interval is extended by buffer minutes after its end (travel time before the next meeting). Nothing is added before its start.
• Busy time outside window is irrelevant; busy time that straddles a window edge blocks only the part inside the window.
• Busy intervals that overlap or touch (one ends exactly when the next begins) leave no gap between them.
• A free gap is a maximal stretch of the window with no busy minute. It qualifies if its length is at least length minutes (exactly length qualifies).

Input: calendars, a list of lists of interval strings; window, a string "HH:MM-HH:MM"; length, the meeting length in minutes; and buffer, the number of minutes added after every non-empty interval.

Output: a list of strings "HH:MM-HH:MM" (zero-padded), one per qualifying gap, in chronological order. Return the whole gap, not just the first length minutes of it. If no gap qualifies, return [].