I just noticed this same thing is happening to our calendar files. The date holds, but the start and end times keep changing on their own. I'll set the start time to 8:00am and the end time to 11:00am, click save...and then when clicking back into it the start time has changed to 12:00pm and the end time has changed to 3:00pm.
Our Marketo instance is set to eastern standard as is the event's date/time.
@Scott have you checked the calendar files for any of your upcoming events? The day after posting my comment I logged back in on a different computer and reset the times times on our upcoming event .ics files to what they should have been (and originally were).
Checked them again a few times over the next couple of days and they appear to be holding/saving correctly. Hopefully this was a temporary glitch and yours are back to normal too. Definitely something to keep a close eye on though.