You’ll need to update the caldera-forms/fields/calculation/field.php
file.
Change this line (~62)
$binds_vars[] = $fid . " = parseFloat( $('[data-field=\"" . $fid . "\"]').is(':checkbox') ? checked_total_" . $field_base_id . "($('[data-field=\"" . $fid . "\"]:checked')) : $('[data-field=\"" . $fid . "\"]').is(':radio') ? $('[data-field=\"" . $fid . "\"]:checked').val() : $('[data-field=\"" . $fid . "\"]').val() ) || 0 ";
With this :
if($cfg['type'] == 'date_picker') {
$binds_vars[] = $fid." = ((new Date( $('[data-field=\"".$fid."\"]').val() )) / (1000 * 3600 * 24))";
} else {
$binds_vars[] = $fid . " = parseFloat( $('[data-field=\"" . $fid . "\"]').is(':checkbox') ? checked_total_" . $field_base_id . "($('[data-field=\"" . $fid . "\"]:checked')) : $('[data-field=\"" . $fid . "\"]').is(':radio') ? $('[data-field=\"" . $fid . "\"]:checked').val() : $('[data-field=\"" . $fid . "\"]').val() ) || 0 ";
}
This will convert your date to a number of days. So when you substract dates, you’ll get number of days between them.