Fatal error: Uncaught Error: Call to undefined function cal_days_in_month() in /var/www/html/wp-content/plugins/backwpup/inc/class-cron.php:372
Stack trace:
#0 /var/www/html/wp-content/plugins/backwpup/inc/class-install.php(93): BackWPup_Cron::cron_next('0 0 * * *')
#1 /var/www/html/wp-content/plugins/backwpup/backwpup.php(74): BackWPup_Install::activate()
#2 /var/www/html/wp-content/plugins/backwpup/backwpup.php(196): BackWPup->__construct()
#3 /var/www/html/wp-includes/class-wp-hook.php(324): BackWPup::get_instance('')
#4 /var/www/html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array)
#5 /var/www/html/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#6 /var/www/html/wp-settings.php(578): do_action('plugins_loaded')
#7 /var/www/html/wp-config.php(136): require_once('/var/www/html/w...')
#8 /var/www/html/wp-load.php(50): require_once('/var/www/html/w...')
#9 /var/www/html/wp-blog-header.php(13): require_once('/var/www/html/w...')
#10 /var/www/html/index.php(17): require('/var/www/html/w...')
#11 {main}
  thrown in /var/www/html/wp-content/plugins/backwpup/inc/class-cron.php on line 372