I only noted this today and I just upgraded to My Calendar 2.1.4 (but can't be sure if this is the cause), but I'm seeing the dates scramble in the Upcoming Events widget. Some events are correct, but some are advancing 1 day. No pattern that I can see. If I view them in the admin interface they show the correct date (although the sorting seems a bit odd). They display properly in the calendar.
Not sure how long this will be a valid example, but:
shows the differences quite clearly.
I deleted and re-added the widget, just in case. All stylesheets and behaviors are defaults as far as I know.