How to initialize static variables with a function call?

I have this code:

private static $dates = [
  'start' => mktime( 0,  0,  0,  7, 30, 2009),  // Start date
  'end'   => mktime( 0,  0,  0,  8,  2, 2009),  // End date
];

I get the following error:

Parse error: syntax error, unexpected '(', expecting ')' in /home/user/Sites/site/registration/inc/registration.class.inc on line 19

If I change mktime to a string, it works.

Anyone have any pointers?