OK – well the best way to modify something like the date/time format is at template level, rather than altering the custom field data directly.
First thing to do is read up on theming and template overrides in particular, so that any changes you make are done safely and in an update-resistant manner.
What you are probably going to want here is to edit your custom list.php and find this line of code:
echo tribe_get_start_date();
And try changing it to:
echo tribe_get_start_date(null, false);
And that should stop the time from also being displayed.