在数据表编辑器中使用数据库函数'NOW（）'

I would like to set a timestamp value indicating when a particular operation was undertaken. That will involve me setting a time value before creating a record, something similar to this.

->on( 'preCreate', function ( $editor, $values ) {
        $editor
         ->field( 'datetime_created' )->setValue( '2014-03-19 11:44:40' );
        } )

I use php code to set the datetime variable. I would like to use the database function NOW() instead.

I presume it would be something almost similar to this:

->on( 'preCreate', function ( $editor, $values ) {
        $editor
          ->field( 'datetime_created' )->setValue( 'NOW()' );
        } )

But off course its now working.

What options does the datatable editor give me to effectively use database functions?