Ok it has to be a time-zone bug. Because when you enter calendar event data it allows you to choose the timezone, so vbulletin must be doing some additional calculations based on the user's own timezone selection, and the timezone for which the event was added.
Pacific Time (for both event and user setting):
August 14th 8am - 4:30pm ---works fine
August 14th 8am - 5:00pm ---links the 15th as well
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