It looks like some of my users were entering dates by hand instead of using the date picker. I had not accounted for that!
I doubt I can train them to use a standard date format, so instead I changed:
<?php echo $property['movein_date']; ?>
to:
<?php echo date("d-m", strtotime($property['movein_date'])); ?>
Thanks for pointing me in the right direction, Maria!
