Return TRUE if value has been changed and old value was empty.

Return value

Bool

Full documentation

Change status to 'Delivery' when delivery_started -timestamp has been given:

if ( $Order->delivery_started->isInserted() ){
    $Order->status->setValue('Delivery');
}

This is shorthand for:

if ( $Order->delivery_started->isChanged() && $Order->delivery_started->getOldValue() == '' ){
    $Order->status->setValue('Delivery');
}