@TomRutko this should work
$('#ctrl-StartDate').on('change', function(){
flatpickr('#ctrl-EndDate', {
minDate: Date.parse($(this).val(), 'Y-m-d H:i:S'),
altFormat: 'F j, Y - H:i',
dateFormat: 'Y-m-d H:i:S',
defaultDate: Date.parse($(this).val(), 'Y-m-d H:i:S')
});
});