nextcloud-server/apps/calendar/js/settings.js

$(document).ready(function(){
	$("#timezone").change( function(){
		OC.msg.startSaving('#calendar .msg')
		// Serialize the data
		var post = $( "#timezone" ).serialize();
		$.post( oc_webroot + '/apps/calendar/ajax/settimezone.php', post, function(data){
			OC.msg.finishedSaving('#calendar .msg', data);
		});
		return false;
	});
});