I have a simple event calendar which I only want to show the upcoming dates. I’ve tried some of the filter solutions suggested in the forum but keep on getting errors.
How do I correctly add a filter to the following query?
$entries = $calPage ? $calPage->children()->unlisted()->sortBy('calendar_event_date', 'aesc') : null;
You need to add filterBy() and then filter by date, see also Filtering collections | Kirby CMS
Filter before sorting
Fab, “date >=” was catching me out. I have it working now 
$today = date('Y-m-d');
$entries = $calPage ? $calPage->children()->unlisted()->filterBy('calendar_event_date', 'date >=', 'today')->sortBy('calendar_event_date', 'aesc') : null;