Article, response and visitor writing

Fatal error: Uncaught TypeError: date(): Argument #2 ($timestamp) must be of type ?int, string given in /export/depts/fass/wp/demand/wp-content/themes/demand/loop-demand_event.php:30 Stack trace: #0 /export/depts/fass/wp/demand/wp-content/themes/demand/loop-demand_event.php(30): date('d/m/Y', '05/18/2017') #1 /export/depts/fass/wp/demand/wp-includes/template.php(812): require('/export/depts/f...') #2 /export/depts/fass/wp/demand/wp-includes/template.php(745): load_template('/export/depts/f...', false, Array) #3 /export/depts/fass/wp/demand/wp-includes/general-template.php(206): locate_template(Array, true, false, Array) #4 /export/depts/fass/wp/demand/wp-content/themes/demand/single-demand_event.php(20): get_template_part('loop', 'demand_event') #5 /export/depts/fass/wp/demand/wp-includes/template-loader.php(106): include('/export/depts/f...') #6 /export/depts/fass/wp/demand/wp-blog-header.php(19): require_once('/export/depts/f...') #7 /export/depts/fass/wp/demand/index.php(17): require('/export/depts/f...') #8 {main} thrown in /export/depts/fass/wp/demand/wp-content/themes/demand/loop-demand_event.php on line 30