Thanks to Joey Kudish and Jonah at Modern Tribe, Inc., I’ve converted my original hacked together code to add an alarm to a calendar event created using the Events Calendar PRO WordPress plugin into a plugin of my own. You can see/follow the original discussion on the Modern Tribe forum.

This plugin requires the Events Calendar PRO plugin. You will have to create an Additional Field from The Events Calendar Settings page.

You can then download, install and activate the The Events Calendar PRO Alarm plugin. If/when this functionality ever becomes part of Events Calendar PRO simply deactivate the plugin.