Comment 7 for bug 1542532

The problem is that an all day event has an end time on the next day, therefore it also is considered to be in the Upcoming category. This shows the Upcoming category date range and then all-day event details:

=== events. category start: "Thu Jun 16 00:00:00 2016"
=== events. category end: "Fri Jun 17 23:59:59 2016"
=== events. event start : "Wed Jun 15 00:00:00 2016 GMT"
=== events. event end : "Thu Jun 16 00:00:00 2016 GMT"

Note that the event ends at the start of June 16, which is in the category range.