Checking array sibling values

I am working on a calendar in PHP and I have an array of all of the days in a month with their availability status. This data is coming from an external XML file which is being parsed and converted into a PHP array for processing. The array looks something like this;

$days = array(
     '30-Oct-2015' => array(
         'available' => true
     ),
     '29-Oct-2015' => array(
         'available' => false
     ),
     '28-Oct-2015' => array(
         'available' => false
     ),
     '27-Oct-2015' => array(
         'available' => false
     ),
     '26-Oct-2015' => array(
         'available' => true
     ),
     '25-Oct-2015' => array(
         'available' => true
     ),
);

And I want to add an additional key, alongside available, to indicate how far away the next true value is. e.g

$days = array(
     '30-Oct-2015' => array(
         'available' => true,
         'daysUntilAvailable' => 3,
     ),
     '29-Oct-2015' => array(
         'available' => false
         'daysUntilAvailable' => 2,
     ),
     '28-Oct-2015' => array(
         'available' => false,
         'daysUntilAvailable' => 1,
     ),
     '27-Oct-2015' => array(
         'available' => false,
         'daysUntilAvailable' => 0,
     ),
     '26-Oct-2015' => array(
         'available' => true,
         'daysUntilAvailable' => 0,
     ),
     '25-Oct-2015' => array(
         'available' => true,
         'daysUntilAvailable' => 0,
     ),
);

How would I go about doing something like this in PHP?

via Chebli Mohamed