Sorry that I didn't get all details correctly, as I said I was confused about the different views.
Yes, I found output.class.php and it has the capability, but there is no option for a backend user as you know. And there is those span elements, but dates are also in span tags with the exact same class name and if there are start and end date as well as items with only start date in the same list, it becomes absolutely impossible to do with CSS. Also, the comma can't be hidden as it's not in any tag. This is an example for what JEM produces for one list item:
<span class="jem_date-1">5. Mai. 2017</span>, <span class="jem_time-1"> 18:00</span> - <span class="jem_date-1">7. Mai. 2017</span>, <span class="jem_time-1"> 15:00</span>