Funnily enough, as it is a Widget in the Calendar app, and it's the "Upcoming events", there is an obvious file to look at as a starting point...
application\calendar\widgets\upcomingEvents.php
And in there, the following line gets the list of events:
$events = \IPS\calendar\Event::retrieveEvents( $_today, $endDate, ( $calendars === NULL ? NULL : !\is_array( $calendars ) ? array( $calendars ) : $calendars ), $count, FALSE )
Which directs you to applications\calendar\sources\Event\Event.php, where the retrieveEvents function lives. And it contains 264 lines of code (or more, or less, depends on your IDE and settings) to look through. And that takes more time to do...and would require it to be sent to the developers, not the front-line support guys, to work on and will take them away from already existing work they are doing on v4.6.
If I have the time later, then I'll look through that code but NO promises at all being given here.
Don't set a precedent.