Trouble importing event dates

I’m attempting to import events into Events Manager using the “WP Ultimate CSV Importer” plugin (I can’t find any documentation on the import/export functionality of EM and when I attempt to export events to see the format it just says ERR_INVALID_RESPONSE, thus I’m using another plugin). I’m importing the event start and end dates as the custom fields “_event_start_date” and “event_end_date” respectively, but when I open the imported events in admin view they date fields are empty. I import the event start and end times the same way and have no problem. Any ideas what could be wrong?

https://wordpress.org/plugins/events-manager/