I searched about it with reference to other plugins/PHP in general. Many people have suggested to declare $calls as an array first before using it. Something like the below I suppose:
$calls = array();
$calls = get_option(‘newsletter_diagnostic_cron_calls’, array());
$calls = time();
I tried and it worked. Crons are going ahead at least. Not sure if it breaks any internal plugin functionality though.
Would be great if you can check.