If you call method that does not exist in Field -class but does exist in eDate class, call is delegated to getDateReference() object. ie all changes to date value will be mirrored to value in field.

Return value

Object eDate

Full documentation

Add 14 days to delivery date:

$Order->delivery_date->addDays(14);

is shorthand for:

$Order->delivery_date->getDateReference()->addDays(14);