Event entry date reverts to 1970 when time is set

I am trying to create an event (Getting Physical—White Mountain Geology, Fossils, and Seismology). When I add a start time and publish, the date of the event changes to 1970-01-01 and populates every date of the calendar. I have this site on two test servers and it does not occur on those sites, which are running the same themes and plugins.