Better UX: only display the Modified date when different of Published one:
& using your favorites Date & Time formats as defined in General Settings:
<?php if ( ($date_pub = osc_item_pub_date()) !== '' ) {
echo '<strong class="publish">Annonce publiée le '. osc_format_date( $date_pub, osc_date_format().' à '.osc_time_format() ).'</strong>';
}
if ( ($date_mod = osc_item_mod_date()) != NULL && ($date_mod !== $date_pub) ) {
echo '<span class="update"> et <strong>modifiée</strong> le '. osc_format_date( $date_mod, osc_date_format().' à '.osc_time_format() ) .'</span>';
} ?>