In our system we are using your library to get occurrences of Events. Our Events Start/End datetime use TimeZone, to support the Daylight time saving. According to RFC, the Until must be UTC, which is. Since the Until is a punctual date, being UTC is not an issue to us.
But, when we are getting occurrences, the until seems to be interpreted in the Event Time zone and not in UTC. Outlook and Gmail doesn't ignore the TimeZone and translate the UTC time.
You will find in attachment 4 calendars.
The occurrences end threashold in you library is shown by the calendars "115959Z" and the calendar "120000Z".
The occurrences end threashold of gmail and outlook is shown by the calendar "165959z" and the calendar "170000Z"
Theses test are valid in the "Eastern Standard Time" time zone, which is -5 (or -4 when in daylight saving time).