macsteve: right! I’m building classifieds site too 
I’ve extended SAEF following - maybe will be interesting for you too:
PHP variables
<?php
global $LOC;
$future_time_7={current_time}+604800;
$future_time_14={current_time}+1209600;
$future_time_30={current_time}+2419200;
$future_time_60={current_time}+5184000;
$expiration_7 = $LOC->decode_date('%Y-%m-%d %g:%i', $future_time_7);
$expiration_14 = $LOC->decode_date('%Y-%m-%d %g:%i', $future_time_14);
$expiration_30 = $LOC->decode_date('%Y-%m-%d %g:%i', $future_time_30);
$expiration_60 = $LOC->decode_date('%Y-%m-%d %g:%i', $future_time_60);
$label_7 = $LOC->decode_date('%d.%m.%Y', $future_time_7);
$label_14 = $LOC->decode_date('%d.%m.%Y', $future_time_14);
$label_30 = $LOC->decode_date('%d.%m.%Y', $future_time_30);
$label_60 = $LOC->decode_date('%d.%m.%Y', $future_time_60);
?>
SAEF select field - user can choose lenght of displaying your insertion
Expiration time<br />
<select name="expiration_date">
<option value="<?php echo $expiration_7; ?>">1 week (to <?php echo $label_7; ?>)</option>
<option value="<?php echo $expiration_14; ?>">2 weeks (to <?php echo $label_14; ?>)</option>
<option value="<?php echo $expiration_30; ?>">1 month (to <?php echo $label_30; ?>)</option>
<option value="<?php echo $expiration_60; ?>" selected="selected">2 months (to <?php echo $label_60; ?>)</option>
</select>
If you want, we can share own solutions. I’ve solved almost all problems how to turn EE to classifieds site.
