Eloquent - select date diff

Eloquent - select date diff

Here is my code (Laravel 4 app):
Reservation::select(array(
'obj_id', 'name', 'surname',
'date_from', 'date_to',
'days',
'status', 'slug', 'payable'
))
My question is how to change 'days' to get real diff between date_from and
date_to?