Hook for when user logs into admin area.

I’m looking for a hook that will run once and once only when someone logs into the admin area. However I do not want the hook to run, after the intial login, when a new page within the admin area is loaded.

From the description I thought admin_init should work like this:

admin_init is triggered before any other hook when a user access the admin area.

However in the implementation I have shown below, my plugin options are being updated everytime a new page loads in the admin area.

add_action( 'admin_init', 'zmb_plugin_admin_init' );
function zmb_plugin_admin_init() {
  update_option('zmbShownSalesFrom','2013-01-01' );
  update_option('zmbShownSalesTo',  date('Y-m-d'));
}

Can someone suggest a way to have these options update only on the intial login?