Right now, activities are removed from the map once their start time passes. Sometimes activities (like farmers markets) don't require you to show up by a particular time, so when you go to find the activity on the map to navigate, it doesn't exist. If we filter by end time instead, they will stay there until the activity is done.

A case could be made for keeping activities on the map based on their date and ignoring time completely. If that was the case, fading already started activities may be a good idea.