date field=calculating time

you mean like that?
<?php
// Modular item layout
// If customizing via CSS rules or JS scripts is not enough, then please copy the following file here to customize the HTML too
include(JPATH_SITE.DS.'components'.DS.'com_flexicontent'.DS.'tmpl_common'.DS.'item_layouts'.DS.'modular.php');


?>

<?php
$field_id = 18; // CHANGE THIS

if ( !empty($items->fieldvalues[$field_id]) )
{
$nowdate_secs = time();
$fielddate_secs = strtotime($items->fieldvalues[$field_id][0]);

$elapsed_secs = $nowdate_secs - $fielddate_secs;
echo $elapsed_secs / (60*60*24*365.25);
echo " years old <br/>";
}
?>

Hello

$item not $items

<?php
// Modular item layout
// If customizing via CSS rules or JS scripts is not enough, then please copy the following file here to customize the HTML too
include(JPATH_SITE.DS.'components'.DS.'com_flexicontent'.DS.'tmpl_common'.DS.'item_layouts'.DS.'modular.php');


?>

<?php
$field_id = 18; // CHANGE THIS

if ( !empty($item->fieldvalues[$field_id]) )
{
$nowdate_secs = time();
$fielddate_secs = strtotime($item->fieldvalues[$field_id][0]);

$elapsed_secs = $nowdate_secs - $fielddate_secs;
echo $elapsed_secs / (60*60*24*365.25);
echo " years old <br/>";
}
?>

Still the same, no errors no "age"...do I have to edit anything in the fieldsettings?

Hello

set error_reporting parameter to "simple"
in
Joomla Global configuration / Server Tab

then use the code without the if statement:

aaahhhh...we are getting closer!

looks like the time is displayed below the whole template instead of repalcing the date/ Birthday

the second line is because I also insert the code in the html5-item.php. I delete the code and now the is only one time/line displayed