Create a button (link) for an ICS file that is generated on the fly via javascript.

Cleanshooter

ICS Button is a simple plugin that adds a button to the editing toolbar (Tiny MCE) that adds an “Add to Calendar” button to the post or page. This “button” is really just a link that when clicked uses javascript to generate an .ics file that users can download. These files (when opened) provide people with an easy way to add your event to their personal calendar application. ICS files are supported by most major calendar applications including: Google Calendar, Apple Calendar (formerly iCal), GoDaddy Online Group Calendar, IBM Lotus Notes, Yahoo! Calendar, Evolution (software), eM Client, Lightning extension for Mozilla Thunderbird and SeaMonkey, and by Microsoft Outlook and Novell GroupWise.
By default the button uses an image that comes with the plugin. However, since the link is just an image you are free to change the image to another one or add text to it. You can even change the size of the image with WordPress’s built in image scaling feature.
Development
<ul>
<li>Repo: <a href="https://github.com/cleanshooter/ics_button" rel="nofollow ugc">GitHub</a></li>
<li>Issues: <a href="https://github.com/cleanshooter/ics_button/issues" rel="nofollow ugc">Tracker</a></li>
</ul>
Credits, Thanks and Inspiration
<a href="https://github.com/nwcell/ics.js" rel="nofollow ugc">ics.js</a> – by Travis Krause

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